Well, I've just spent a happy 4 hours stripping down my carbs, and rebuilding them with a spares kit from Burlens.
There was a washer in the kit which didn't seem to fit the carbs anywhere, but hey...  everything else went in, which I thought was a good result.
Question though.  The instructions show a washer at the upper end of the jet bearing, and there was a spare washer provided in the service kit.  However, the carbs as they were when I stripped them didn't seem to have this washer fitted.  I've added the washer in as I've rebuilt, but now I'm thinking maybe I should have left it out.  Any thoughts?
The carbs are SU HIF6s.