Ok thanks, guess I have to stick to vector maps then. I heard about an open-source UK vector map that is being kept up-to-date - can't find a link for it now though - any ideas?

I had the NWAtlantic BlueChart V2 working on it today while out sailing Now downloading the UK TOPO V2. Hopefully I can combine the two map-sets using GMAPTool - the result should be less than 4GB. Are there any problems combining maps and charts? They are pretty much the same thing really, though not sure on tidal data etc.