I've not managed to find paricularly well detailed info on it but I
assume that if they were to build a stadium that was 60% more than
their existing capacity, then they'd be on the hook for some pretty
dramatic improvements to the local transport?

I know Arsenal have got away without doing a whole great lot but
getting to Highbury was never that tough anyway and options to go NSEW
within a relatively short walk, and WHL has always been a bit of a
nightmare to get to and from, with the only real option being to walk
down the High Road to Seven Sisters. Is there going to be a proper
extension to the Victoria Line up to Northumberland Park?