Tom21 - probably the tune many of us here have whistled. I am one and here is what I have found about some of the suggestions above (but this is just me)
Metropolis - there is on average 250-270 ads placed by guys. General 4:1 compared to women posting. I have had some hits, but the women can be flighty or 'looking for a friendship'. Worth dropping a line in the water, though.
Cupid - most looking for the man of their dreams. I was on it for a month but it drove me crazy and the question-filling in thing was addictive.
Universal - costs, but have not tried it. Might.
Tried one sugar baby site - my only hit ended up being someone who just joined the same company!! Hilarious! Too close for comfort.
I am probably too old and too worried about being spotted to do the Deai Cafes, but they sound interesting.
If you just want to play with no dramas, etc. then P4P might be the way to go. What you have to decide then is your Yen limit and what sort of play you want. Plenty of advice on this site.
Serena - I will be in London early August!!