I am scratching now I am thinking about it!
I have brushed Noah loads with the Furminator (fine teeth) and not found anything else. Vets are going to give me Stronghold when I go in tomorrow (my day off): the flat needs a good clean too. I agree about being safe rather than sorry, I wouldn't want to risk Noah suffering and I once moved into a house with fleas.
The more I look at it the more I think it looks like a flea, just seems strange to have brought it all the way home from my friend's flat; I only go through the courtyard on the way out not the way in. Suppose I could have brought eggs in?? Yuck yuck yuck. I have texted my friend to warn her.