Set in London - group of friends - female uses British Library to solve mystery - fancy dress party? Read 1995-1997.

[Tracey]

I read this book between 1995 and 1997. It was set in London and involved a group of friends - there was a mystery to solve and the female lead? went to the British Library to find clues. I recall a fancy dress party and an attic flat in a nice area of London. I think it was set in the 1960s - 1980s