Yeah one of the reasons I don't want to get the VoIP account and do it through the POTS line is that i know of a few other people who want to do something similar, and who just wont entertain the idea of changing phones lines over to voip etc.
I've also been looking at the likes of the rasberrypi + asterisk setup. Am i correct in thinking when coupled with a FXO adapter might work as a solution? (obviously when configured correctly, im under no illusion that Asterisk can require a fairly in-depth setup) My concern is that when the call comes in it would be routed to Asterisk and the existing analog phones in the house wouldn't ring.
The ATCOM IP01 PBX looks interesting, although as mentioned the Fritz!box has all this + modem/router functionality. (Not overly concerned with the price of the Fritz!box, it's within what I'm happy to spend)
I really appreciate the help with all this, thanks guys.