In reply to ti_pin_man:
Hi, I, a joiner, In theory its a straight forward task.
Each loft is different though and without seeing yours I can only offer general advice,
Firstly you will need to use chipboard or floor boards,(chipboard would be my suggestion) to floor the loft space. the last thing you want is to come crashing through your ceiling.
as for the wall itself you are best to use 18mm WBP (30-40 quid per sheet)
and you can fix this directly to the underside of your roof rafters, just make sure you put plenty of screws in it,
I,m not sure what you mean by ceiling currently in there, is the loft already converted.
I hope this helps, I could go into more detail if you would like,
just private mail me.
cheers
Neil