I'm woried that as I'll be 40 when I graduate, with an expected First in Microbiology, I wnt be able to find a PhD placement. Have any other mature stduents had an experiences in this area?

I am doing a PhD in the same lab as a 45 year old who is just starting her PhD. If it's what you really want - go for it! Good Luck ;o)

A man in London has recently completed his PhD at the age of 93, so you shouldn't have too many problems!