The thing is, that is a very convoluted setup and I would argue very uncommon.

Most UK ISPs use PPPoA but the router deals with all that. If you actually have to connect via PPP from the PC then in fact you aren't configured as a router at all, its just acting as a plain modem. Its an aweful configuration as usually then only one computer will be able to connect at a time.

So as people keep saying, you would be far better off getting a good router to do the PPPoE as then you only need to connect your computers and N900 to that router and forget all about PPP. Unless of course you mean public access points in Cafes, Airports, etc force you to use PPP, then I suppose you will need to find a solution. But the fact is hardly anyone is going to need PPP on their N900 so getting help on how to do it will be tricky.