I have managed to find the geographical for London (not sure which one).  You could ring them and ask them to transfer you or ask for the number (they may give you the geographical number without realising it).

The geo number for london is 0207 636 7000.

The website for them (and their different offices) is:-

http://www.dunlophaywards.com/contact.php

You'll see that most offices are already using 0870 but I did find Leeds that wasn't and they gave me the number for London (above) as they didn't know Croydon's number.

At the time of writing this, Internet Archive was down so wasn't able to check.

Update:  Internet Archive never had any entries for the above website.  A Google search reveals that a few companies operate from that address.

Thanks for all your efforts guys. This last number is a direct line to an individual. I've left a message so we'll see what happens...