I only ever use the official HPI check site, normally online or over the phone..they will also send you a certificate if you request it.
Will try and get you a link.
Regards to payment, if buying private, normally cash and take a few people with just incase, leave a deposit and come back with cash later. Never tried CHAPS or cheque so can't comment sorry.
Hope it all goes to plan, I'm looking for a GTI. Again too!