Anyone know where I could get hold of an adapter to convert 6 amp 250 volt ac plug (three skinny pins - Masterpiece, England) to a standard British three-pin ?
It's hard to say without knowing the shape of the pins or their configuration, but it sounds like the old lamp plugs with round pins in a triangle. You're unlikely to find an adaptor for these - just cut the plug off (unplugging it first!) and fit a new plug with the correct fuse.