Thanks everyone for your perspectives. Here's a bit more detail:-
The guy behind me came up so fast that by the time I realized he was there, I did not have time to brake or stop to avoid the pedestrian in front of me. I already rang my bell several seconds before. If I had used my hand to signal the fast rider to slow down he would have just plowed into my arm. So the choices were (i) to go to the left of them (that means riding into the river) (ii) go with original intention and pass between the pedestrians but change my line so I maximize room for the fast guy, (iii) ignore the fast guy and just pull to the middle of the MUP so he plows into the river or the pedestrians or me. I did (ii) and went close. The rider at the back presumably got out his feeler gauges and quickly checked his clearance each side as went through. It was dark.