Thanks everyone for giving me things to try.
I upped the lease time to 1 week and changed the pool to 192.168.1.125 and assigned the humax box to 192.168.1.126.
Strangely it ended up 192.168.1.42 but it still worked so i left it.
Unfortunately today it has stopped working again and the ip address has gone back to 192.0.2.100.
The old router has gone also.
I know I'm a pain sorry, I'm surprised nobody else has the same box/router setup as me with no problems.
Thanks again