An Internet Archive reveals they have been using 0870 for a couple of years.
I did manage to find out they have the same address as ISP
v21 and
GetFresh (apart of the v21).
A 192.com search gave a geographical for v21 as 01276 375 93.
Therefore, try 01276 375 93 (it was just ringing out at time I wrote this) and providing its not a household number then play dumb and ask them are they Fast4 as Directory Enquiries have just forwarded you on to them. If they try and give you their number (which near enough proves they are one and the same company) then ask could they transfer you as your calling from your mobile and DQ have forwarded the call which means you are paying a fortune.
You could also try a number nearby that one like 01276 375 90 or 01276 375 95, etc.
Let us know the outcome regardless because if it is v21 I'll add it to the database.