Sounds like you are in the UK (Ordnance Survey) but I wrote a mashup to combine mapping layers and provide other utilities (slope analysis etc) for my own planing of off-road bike, hike and ski trips:
It mostly covers the US and there are a lot of legal issues surrounding the different layers meaning it is difficult to run it as anything but a free site which may be why more people don't do this.
http://www.hillmap.com
It mostly covers the US and there are a lot of legal issues surrounding the different layers meaning it is difficult to run it as anything but a free site which may be why more people don't do this.