Sir Arthur Bonsall: Linguist who broke wartime aircraft codes at Bletchley Park and went on to serve as Director of GCHQ

In retirement Bonsall took up the cause of preserving the Bletchley site for posterity

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Love, and the prospect of lifelong delving into the deepest of secrets, blossomed at Bletchley Park for a young Cambridge German and Russian linguist and the girl he could see from the window of his decrypting hut. From his makeshift office Arthur Bonsall, sitting on his folding chair before his trestle table in Hut 4 of the Government Code and Cypher School, writing out in pencil lines of German airmen's code, had a view into the panelled room inside the government-requisitioned great house where only yards away she toiled over her Italian ciphers.

This was 1940, Britain's year of fearing Operation Sea-Lion, the expected Nazi invasion in the wake of the fall of France; yet neither the coming Blitz, nor the crumbling of Britain's Far Eastern empire the following year could spoil his happiness, and he got married.

The union would produce seven children and sustain him all the way to the top: he was destined to be the knighted head of the top-secret Cold War organisation that would succeed ramshackle Bletchley, and in the satellite age would be known as the Government Communications Headquarters, GCHQ.

For the moment, however, the new recruit, whom the army had turned down because of a barely detected heart murmur, was a subordinate to a memorable boss. Josh Cooper, the eccentric but brilliant head of Bonsall's area, the Air Section, was a man who when faced with the smart click-heels and Nazi salute performed before him by a captured German Luftwaffe pilot, had scandalised his staff by echoing the favour, his arm involuntarily rising and his voice exclaiming "Heil Hitler!" before he came to his senses and tripped over in confusion.

Cooper, despite an array of wild mannerisms, had great linguistic, mathematical and radio skills, and gave Bonsall his first promotion, from Junior Assistant to being in charge of the Small Cypher sub-section which would handle as many as 2,000 messages a day.

The Air Section worked on breaking aircraft codes and secret call-sign systems, termed "low grade" but in fact highly useful, and its first success was solving coded German weather signals. Zenit long-range weather reconnaissance aircraft were allowed to escape being shot down by the RAF's Coastal Command so that the signals could continue, to Britain's advantage. Bonsall remained convinced that the Germans knew about British plans for the ill-fated Dieppe Raid of August 1942 because the German codes suddenly changed on the day of the operation.

He spent some time at the RAF's "Y" intelligence listening post at Cheadle in Staffordshire and came to the view that his type of Air Section (or "GAF" – German Air Force) work was chronically short-staffed because of the demands of Bletchley's other concern, the breaking of German "Enigma" codes. Towards the war's end Cooper handed the Air Section over to Bonsall as head.

Educated at Bishop's Stortford College and St Catharine's College, Cambridge, Bonsall had been recruited through Martin Charlesworth, a Fellow of St John's College and university Proctor, or disciplinary officer. Charlesworth invited Bonsall for an interview after hearing about him from his landlord, who also worked as a university policeman, or "buller", reporting to the proctors. "There were two men there, they weren't introduced," Bonsall recalled of his first encounter with the organisation to which he would devote his life. "And all they said, virtually, was was I interested in confidential war work... I immediately said 'yes'."

Bletchley, based in and around a former 19th-century Liberal MP's home near present-day Milton Keynes, was closed down after the war and Bonsall, by then interested in the emerging tensions with Britain's erstwhile ally the Soviet Union, moved first to offices in west London then in 1952 to Cheltenham, where GCHQ has remained ever since. In 2003 it moved from the two secret buildings he knew, at Oakley and Benhall, to the distinctive circular office building of today. Throughout Bonsall's career the organisation's identity was hidden, the Government never admitting to its existence. It was known locally as "the Foreign Office in Cheltenham". He rose through the hierarchy, being for a time head of "J" Division, which handled Soviet "sigint", or signals intelligence.

GCHQ by then had access to sigint gleaned by its much larger equivalent in the US, the National Security Agency, through the UKUSA agreement of 1946. GCHQ's staff grew from the few hundred it inherited from Bletchley to about 8,000 by the mid-1960s.

Britain was able to offer the US the global reach of listening posts established across its colonial empire. As this dwindled, agreements were made with newly independent countries – or ruses resorted to, as when on at least one occasion radio masts were secretly installed disguised as flagpoles. Britain also sold old Enigma code machines to Third World countries, leaving them unaware that the Cheltenham staff could read their every encryption.

In 1973 he took over as GCHQ's sixth chief since the Second World War, and the last who, like all his predecessors, had worked at Bletchley. In that year he helped restore British relations with the NSA after Britain's refusal to let the US use its Cyprus listening post during the 1973 Yom Kippur War had caused fury. The US Secretary of State Henry Kissinger had ordered the suspension of intelligence co-operation until told the UKUSA agreement precluded him from doing so.

It was during Bonsall's directorship that the KGB mole Geoffrey Prime began work at Cheltenham. He served there in 1976 and 1977. Bonsall retired in 1978, and Prime's espionage was uncovered only in 1982, and his trial that year – he was sentenced to 38 years in jail and released in 2001 – pushed GCHQ's existence into public knowledge. A dispute over whether GCHQ staff could be trade union members drew further attention in 1984.

In retirement Bonsall took up the cause of preserving the Bletchley site for posterity, and also of contradicting the idea that Bletchley Park was "all about Enigma." His brother and only sibling, Frank Bonsall, a mathematics professor, died in 2011.

Arthur Wilfred Bonsall, codebreaker: born Middlesbrough 25 June 1917; CBE 1957, KCMG 1977; married 1941 Joan Wingfield (died 1990; three daughters, four sons); died Cheltenham 26 November 2014.

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