The valve lifter works fine now, John, though the engine needs a good bit of running before I can confirm that the gasket is sealing properly. The material I used for the gasket is nitrile cork. The first sheet I bought was much coarser grained than the original gasket, so I managed to get some denser, finer grained material from a firm called Munsch in Macclesfield. The usual ebay nitrile cork has a density of around 500kg / cubic metre, the stuff I ended up with is about 800 kg. It is also slightly harder and slightly less compressible, similar to a Bukh gasket.
I'm still puzzled, though, how it can be possible to screw the rocker cover down enough to put the lever out of line with the shaft. Page C6 in the workshop manual shows a cross section of the rocker case and its fixing. In theory you tighten the cover nut down fully into the stud, and this should automatically compress the gasket adequately, and at the same time align the shaft and lever. It shouldn't be possible to force the rocker cover down so far as to misalign them. I'll give these chaps you mention, Kirby, ( for which thanks) and see what they say. Ybw mention them too, John, in response to your posting. Again, thanks.
In case it's of any use to anyone else (it's too late for your purposes, Tony) I was able to check the decompressor lever on my DV36 yesterday. It turns almost 90 degrees. I'm pleased to report it actually works. I obviously wasn't pushing it assertively enough when I tried it previously.
Curiously the DV36 (and presumably the DV48) has the decompressor shaft on the other (port) side of the rockers to Tony's DV24, and the operating lever turns anti-clockwise.
Conclusion! I've spoken to Marine Enterprises who were brilliant. They knew exactly what I was talking about and have come across the problem that over-tightening the rocker cover does to the valve lifter operation.
The return spring on the DV24 does indeed rest against the inside of the cover, not against the head. Regarding screwing down the rocker cover, they say just do it enough to stop oil leaks, and definitely not so far that the decompressor action is affected.
So, if your valve lifter won't turn through its 90 degrees, try replacing the rocker cover gasket and go easy on retightening the cover.