Well after all that, I've had to rollback to Infinitecoin version 1.8. It looks like version 1.85 is refusing connections from p2pool (or, from what I can gather, anything that it doesn't recognise as another version 1.85 of the Infinitecoin wallet) . I see that the other P2Pool is currently out of action, which I guess is probably because of this same problem (see Jude Austin's post at https://bitcointalk.org/index.php?topic=356065.msg3881370#msg3881370 )

I'll raise this with TECSHARE as this really needs to be fixed in version 1.9.