Rather than explain that I had not only lived and worked in London for a decade, but also was a civil servant who had lost her way a couple of hundred yards from her office, I smiled and said I'd been here since Tuesday and liked London very much.
I'm truthful by nature, but I've been driven to protective lies to save people from being upset: many become personally affronted when I explain I can't remember if it was Salisbury or Swindon I went to last weekend. Chaps - with their passion for 'Did-you- take-the-A617-or-the-B204?' conversations - tend to insist that all I have to do is try.
The most hideous example of this tyranny occurred when a German asked me if I had ever been to his native land. Only once, and just for two days, I said.
''Where did you go?'
'I can't remember.'
Incredulousness. 'Sorry, I've a bad memory for places.'
'Well, where was it? North, south, east or west?'
'I'm sorry,' I whimpered, 'I flew.'
This was one of those dogged Germans. 'Were you in a city or in the country?'
''What kind? Old? Modern?'
'I don't know. I was at a trade fair. Now tell me what you think of London.'
'No, no. Not until we find out where you were in Germany. You will recognise the name when I say it. Munich?' It wasn't. And it wasn't Berlin, Bonn, Frankfurt or Hamburg either. 'Dresden?'
But a flicker of recognition had surfaced. 'I think it began with a 'D'.' As he opened his mouth to say 'Dusseldorf' - which indeed proved to be the right answer - my brain disconnected completely and I burst out triumphantly with 'Dachau]'
But although nowadays in such circumstances I would deny ever having been to Germany, or say yes instantly to Munich, I am constantly asked questions which require the truth. 'Where was that political conference you attended last spring?'
'Oxford or Cambridge.'
'Where does Josephine live now?'
'What do you mean? You visited her last month.'
'Well, I went from Paddington and she picked me up at a station whose name I don't remember.'
Then, placatingly: 'I think it began with a 'C'.'
Then there are dates. Simply because I'm a historian, people ask me impossible questions all the time about wars and reigns and anniversaries - and, endlessly, these days, about Ireland. When was the Battle of the Boyne, the Ulster plantation, the foundation of the Orange Order, the split in the IRA? And travelling in Ireland with British or American friends produces: 'What happened here? When?'
I can't even say, 'Not my period, I'm afraid', for it usually is and frequently I have written about it. Indeed my first book was An Atlas of Irish History, and as it contains every fact anyone in their right mind could want to know about Irish geography or history, I use it frequently myself and then promptly forget what I have rediscovered.
My friends have come to view my disability as they would a wooden leg - they mock me but compensate for my deficiencies. For instance, one old friend can provide me with every date of significance in my life. And those I travel with have been known to write down for me a list of places I've just been to and what I've seen there.
With age and increasing assertiveness, I apologise less. Having been nicknamed 'the non-retentive historian', I introduce myself thus if new people start asking difficult questions. My defence is that one's memory is finite in capacity and I remember lots of useful information; I forget only what I can look up. As long as I can read books and maps, I can function perfectly well. I understand history, I just don't remember when it happened. I know what sort of a chap Gladstone was, just don't ask me how often he was prime minister.
However, that excuse unfortunately doesn't wash for my other problem area - faces. Once I've met a person a few times, I will remember them for life, but I'm quite capable of talking to someone for three hours at a dinner party on Monday and failing to recognise them at a drinks party on Wednesday. Yet I will remember everything they told me.
I keep as a souvenir a scrap of paper from a trip to America with a friend. In my handwriting, it says: 'Who is that woman?'
In his: 'You silly cow. That is Alison - with whom we drove 300 miles yesterday.'
Beatrix Campbell is on holiday.