It would appear that that number is also used by Fast4 (www.fast4.net), which is (it would appear) a virtual ISP for NJP Services (http://www.njpservices.com/). Their contact number appears to be 0870 225 1058, but their fax number is in the same range as the 0871 number you provided - suggesting that NJP Services provides the customer services for both Zaggle and Fast4 (along with a few others if you Google that number).

Unfortunately, I haven't had any further luck tracking down a number for NJP Services (except for their 0870/0871 numbers). However, someone with more experience in this field might be able to help .

The geographics provided in that thread appear to be in an incorrect format (01xxx xxx xx instead of 01xxx xxx xxx) so I'm not surprised they don't work...

There is also conflicting information as to where this company is based - their website says 61 London Road, Blackwater, Nr Camberley, Surrey, GU17 0AB but their WHOIS says 9 Bellevue Road, Hornchurch, Essex RM11 3SS (which I suspect to be the home address of this Nick person). Google Maps confirms the Essex address is a residential area, but the Surrey one could be correct as it looks to be a building on a main road. I think that is the address to focus on attempting to find a number for...