They actually quote an 0870 number on their site (it seems suspiciously similar to the 07000 number, but it's part of an image, so I can't imagine it's a typo):

08700 70 70 70
08700 70 70 71 (Fax)

If it isn't the number you are looking for, try changing the 07000 to 08700 for whatever number you have.

On an old version of their site, I also found:

Tel: 0870 444 25 45
Fax: 0870 444 25 46

I know these aren't geographical numbers, but they should only cost half of what the 07 number does.

I found versions of their site from 1999 and 2000, but even back then they were using 07000 and 0870.