I am going to get some grinding paste and give it a go. The trouble is that I would have to completely re-route the pipe and have the valve in the wet locker if it doesn't work, and of course (not ideal), I'll only find out that it still weeps when she goes back into the water.
I'll see how I feel when I have had a go at it.
I had a go at Sunrunners' seacocks (Blakes types) last Summer, they were quite pitted when I got all the deposits off but after grinding in there was a clearly visible seal all around the top and bottom of the valves and no leakage when she went back in the water. £250 saved and some effort too so it's well worth doing imo.