Router resetting NAT

[#1]

I have just had to replace my old router with the new Technicolor from Plus. Works OK just to provide internet access. However, I also wish to run a small web server application and have assigned the HTTP server application to one system on the local network (by hostname as offered on the configure page)
That works fine but after a few hours it resets the assignment to another local IP. At the same time, the web interface to the router no longer responds via a browser although service to the network is maintained and the IP for the router will ping just fine.
Plus claim that this is not a supported use of the router and refer me to this forum. (I'm not best pleased!)
Anyone seen this or have an explanation?
ta

[#2]

If the server is on a static IP address, try putting it on a DHCP (automatic) IP address.
IMO these routers have problems with port forward to a static IP address.

[#3]

Thanks for the suggestion but (I should have made it clear in the OP) it is on DHCP at the present - I found the problem with static even before trying to assign an application!
However, I still have the original problem.