I thought I’d sorted this once but apparently not :(
I’d managed to get things sorted so that I could input pound signs and they would appear fine to me and everyone else but if someone else typed a pound sign I got a question mark.
I think what was happening was that it was taking UTF-8 input and converting it to latin1… not entirely sure.
Anyway for those suffering the same issues take a look at The ultimate guide for UTF-8 in irssi and GNU/Screen