Neither Mr Hamilton nor Mr Colvin, the MP for Romsey and Waterside, registered paid-for consultancies with Strategy Network International, the firm specifically created to lobby against economic sanctions and as propagandist for Unita, the Angolan opposition group, and for the so-called 'transitional government' of Namibia set up in defiance of UN resolution 435 on Namibian independence.
In Mr Colvin's case, the omission from the Register of Members' Interests was an 'oversight'. Mr Hamilton has argued that his involvement of less than a month before becoming a Government whip was too short, because MPs are allowed four weeks to make their entries.
That can be contrasted with, for example, the relative openness of the entries by Marion Roe, Tory MP for Broxbourne, in the annual register for the year ended 8 January 1990. Included were the following overseas visits: 'February 1989, to South Africa for three days, sponsored by the South African Coal Industry; 11-18 June 1989, to Angola, funded by Unita.'
Both these trips would have been facilitated, directly or indirectly, by SNI. A major client was the South African Chamber of Mines, and the export of vast quantities of cheap South African coal was being hampered by economic sanctions.
Mr Hamilton recorded in the register a visit as a guest of the chamber of mines in February 1988, but apparently deemed his much more direct link to the lobbying firm, revealed by the Independent on Monday, as not worthy of mention.
Given Mr Colvin's close links with South Africa, his lapse of memory is equally puzzling. One of his past activities was to identify sympathetic MPs who might be interested in what came to be called the 'Bop run' - trips, generally all expenses-paid, for hand-picked Tory MPs to the unrecognised Bophuthatswana 'homeland', one of the dumping grounds for the three million black people evicted from their homes in the former South African government's 'whitening the cities' offensive.
MPs who made the journey between 1987 and 1990 include Graham Riddick, one of the MPs under investigation over the cash-for-questions affair, Andrew Hunter, Colin Shepherd, John Townend, John Watts, now minister for transport, Jack Aspinwall, Sir George Gardiner, Mrs Roe, Nicholas Winterton, Edward Leigh, a former minister, Sir Ivan Lawrence, Martin Smyth and William Ross.
A note of a delegation led by Mr Hunter records that they came 'as opponents of apartheid and with open minds'. Appearing before the Select Committee on Members' Interests in 1989, Ian Findley, who ran Bophuthatswana's London office, was asked: 'Are you satisfied that your government is getting good value for money from visits by British MPs?' He replied: 'Yes, very much so.'
There is no suggestion that the trips were not declared in the register, but allegations of a 'gravy train', paid for by the apartheid regime, abounded.
To the annoyance of the homeland authorities, the form would sometimes be a one-day stop in Bophuthatswana before MPs escaped to a beach holiday in Natal or Cape Town. The usual practice was to offer first-class travel, with the alternative of cashing in a single ticket for two club class seats, enabling MPs to take spouses. A number chose the second option.
Part of the same scene were London dinners sponsored by Unita. One MP guest-list included such figures as Robert Atkins, now minister for the environment and countryside, John Carlile, Mr Colvin and David Atkinson, who led an SNI delegation to observe the Angolan peace process in 1992, a year before the firm closed.
SNI's existence stems from the former South African government's inability to mount an overt offensive against Swapo (South West Africa People's Organisation) over Namibia.
A key figure was Sean Cleary, an erudite but shadowy former South African diplomat, who became director of the administrator-general's office in Windhoek in the early 1980s. Namibian public relations as such were handled by a consultancy run by Sir Trevor Lloyd-Hughes, who was press officer to Harold Wilson when he was prime minister.
SNI was opened in high-cost offices in Storey's Gate, Westminster, in 1985 by Patrick Watson, a former captain in the Black Watch Scottish regiment, and Steven Govier, a Westminster councillor. Sir Trevor employed both men before his contract was terminated in July 1985, having sacked Mr Govier in May of that year. Sir Trevor then mooted the possibility of legal action against the two and accused the new organisation of being controlled by Pretoria. In due course, the right-wing Monday Club activist Derek Laud, now of Ludgate Laud, joined the firm, later recommending the recruitment of Mr Hamilton and Mr Colvin as sympathetic MPs.