well....for north london is:
North London / Enfield TW 40985 515000
i checked the surrounding areas and their not on the list so i guess it all must come under the above......
with regards to the box i have change 1000/2000/4000 no problem to TW area but this was from a c&w area.......i checked coventry and it's pure ntl....so i'm not so sure if this will be the case for a box from a pure area....i'm sure it's not but not 100%...so u'll need to find out if u can change a pure to tw by just changing the net i.d. and freq
hope that helps