Hi. Did you get any recommendations for this? If not, I'd be happy to suggest the people who did ours. They were called Premier Lofts.
http://www.thepremierloftcompany.co.uk/6.html. We used them as they were recommended by friends of ours. I got 5 quotes, and they were probably the middle in terms of costs.
If you contact them, ask for Malcolm Newman and mention my name Alex Moss. They did our loft when I was pregnant (congratulations by the way!) and were very little hassle. 99% of their clients are expectant parents, needing to expand, so he's used to it! We live in Wandsworth Town too by the way, so if you want to come round and see what they did, let me know. Good luck!
Alex
PS: I am currently on maternity leave, and considering whether to go back to my old freelance work project managing in TV (not v simple with childcare to consider) or embark on a new career helping local mums project manage building work. I studied architecture before falling in to telly, and we've had a couple of building projects done on our house so I know how daunting, stressful and time consuming it can be, esp. when trying to do things on a budget - so if you need any further help just holler!