My daughter (also 18 months) has been at Babyroom on Lavender Hill from March, and absolutely loves it there. I went with Babyroom as there were crucially not many children there (one nursery I went to had a max of 16 kids/room!) - I think their max/room is 9, but in actual fact there has never been more than 6 kiddies in my daughter's room. I would say that in terms of equipment etc they haven't got the newest, brightest things but the staff are wonderful; really lovely ladies who absolutely adore my daughter and is adored right back (by all of us!).
I'm pretty sure there's space btw; a couple of 2 year olds left in the summer as they'd moved from here.
Re location - depends where you end up living I guess. I'm on CC Northside and Babyroom is nicely equi-distant between home and Clapham Junction, so is pretty convenient for me.
I would certainly recommend visiting nurseries though before you make any decisions though!
Mdp216 - sorry, don't know any nurseries around Clapham South I'm afraid.