The issue is not with the ip address, but with the actual computer apparantely.
Betfair were not aware of the problem, which to be fair to them is going to be true, as apart form APAT live holding tanks, I can"t think of many situations where this can arise.
Other players have either used another computer, or got a friend to log in and register for them.
Feel free to pm "Betfair_Dave" though, as he may have another suggestion.