So the problem here seems to be a load issue on the BI Studios server which Paypal is attempting to connect to... so maybe the solution here is (and maybe I shouldn't be saying this to everyone!) to try purchasing it at the quietest time possible, bearing in mind the time in the US overlaps on our evening (in Europe) then the best time to attempt it is probably 5.00am to 9.00am UK time. This seems to be backed up by the fact that ArtjomZhdanov got his purchase at 9am, every time I've tried it's been in the evening when the servers are possibly busy.
I will be getting up early and giving it a try tomorrow!