UKH

The screw lock on a small HMS karabiner of mine now feels sticky. It feels as if something soft or rubbery between the screw sleeve and the gate is now out of shape, since it jams ever so slightly but in a soft, damped way, rather than a scratchy gritty way. Are there any rubber parts inside a screw gate (like a washer or seal or gasket of some sort)? Or is this more likely to be damage to the screw thread?

Nothing rubber AFAIK.  I guess you have checked the obvious: dirt ? Worth a quick spray with eg GT85 as well.

As Graham said, your best bet is to clean/flush it with solvents and detergents to try and vacate the pesky invader.  They afterwards, a measured approach to light lubrication.

If it's really cruddy, drop it in a measuring jug full of boiling water with a little bit of detergent and leaving to soak for a while.

then fish it out and screwing/unscrewing under fresh running water for a bit to thoroughly rinse

Dry it as thoroughly you can and leave warm and dry for a day or two to be dry inside

Then gt85 it 

That sounds hard.

In fact it doesn't look or feel dirty at all. It hasn't had a hard life and from the outside it looks pretty good. It is just the screw sleeve that doesn't turn as freely as expected, or as on the other krab of the same model which I have.

I'll give it a try with the washing up liquid at least, but I might wait before boiling it as well.