Decided to go for it in the end. The waterproofs DO still fit, although getting into/out of them is a little challenging these days! Having established that, I kept them on for the run up. One or 2 fairly heavy showers on the way up the A361 but since most of it seems to be roadworks and a 40 limit, it wasn't really a problem - and the waterproofs (a one-piece from Lidl 14 years ago!) are still waterproof, although they don't seal round the neck, so a little got in but not too much.
TBH, I was a little disappointed with the show. Plenty of bikes to see, from a lovely, patinated SS100 Brough Superior (Mrs Nod wants one...) through all manner of stock and custom machinery to a lawnmower engine powered chimera but precious few trade stands. Worth the trip up - but the ride itself was worth that!
Once I'd seen as much as I wanted to (and having bumped into a mate who moved away 30 or so years ago and having a catch up!), I came home down the A377 in the dry. Well, in the dry until I was 3 miles from home, at which point the rain started and the hungry light came on... A mile later, the heavens opened so although I was passing the top of our road, I carried on to fill up since I was already wet! At least I now know that the leather treatment I got at the Devon County Show DOES keep leather jackets as waterproof as possible (seams excepted!) and have been reminded that the lighter, more comfortable for walking round in Kevlar jeans aren't at all waterproof!!! At least I have a full tank and know that I can get 110 miles out of a tankful with some "spirited" riding involved!