Having great difficulty getting the plate to go down on a Smoker's Satisfaction - can pull it upwards when it's in the case, but won't go down. Case is clear of gravel etc. and doesn't appear to bind at all. The bolt is not tight. Is this a coomon problem with this design?

I also have a Smokers, but I haven't met this problem.  I'm pretty sure it has nothing to do with design.

However.... assuming that the pivot is not the issue, there are two possible areas of friction.  First is the pad, or whatever it is called, which is meant to hold the board in position inside the case.  If this has been homemade, e.g. from hosepipe (as it is on mine), and not very well-fitting there can be too much grip in one direction or the other - in my case it is quite stiff to put down but generally easier to raise.  The solution is either to check and trim it a very little or else get a proper commercial product (which I haven't been able to source but must be out there somewhere).

The second possible area of friction is the slot gasket - the rubber, canvas or other material which seals the bottom of the slot either side of the board.  In an old National, I had problems with the gasket being pulled up into the slot when I raised the board.  Trying to put it down again was almost impossible as the gasket 'bunched' and jammed the board.  The answer here is to ensure that the gasket material slides easily over the board and that there is sufficient overlap to ensure it stays outside the slot.

If you find a source for the friction pad inside the case, let me know.  I don't really want to use much more hosepipe in case we get a drought up here in Inverness one of these centuries.

I used the neoprene rubber available from pond/garden centres for my slot gasket and find it very flexible and seems to form a good seal. It's cheap too and has worn well.  I also used mylar on one boat and am not so impressed but maybe I didn't fit it so well.