A few numbers come up:-
In the unverified list, 0207 2103613, comes up which is head office.
A 192.com search comes up with 0117 906 5200 which isn't the helpline but probably a regional office.
Alternatively, their regional offices
here all use geographical numbers except for one which doesn't list a number at all.
My first thought would be (if possible) use a regional office near yourself.
Update:
A further check revealed that prior to November 2002, they only listed regional offices/numbers but since after this date they must have setup a national helpline. As they appear to have set this up and used a 0845 number right from the beginning then its hard to find a geographical especially without knowing where the helpline is based. Is it London in the h/o? It is possible the 0845 is routed to more than one location due to the high number of calls they get.