I looked at OCCS' website on
Internet Archive and between 2002 and 2007 they used the number 020 7261 1017 and PO box address OCCS, P.O. Box 4685, London SE1 6ZB. The phone number is now not recognised which perhaps coincided with the move of location, and I'm wondering if the introduction of the 0844 did as well. So I wonder if the new 01/02 number has never been published.
The
OCCS contact page, as at November 2007, gave the same PO Box in SE1, but telephone 020 7407 3990 and fax 020 7407 3991. The fax is not recognised, but the voice number rings; you could try that (let us know how you get on if you do).
The current address given is (again a PO box):
OCCS
PO Box 219
Petersfield
GU32 9BY