To answer my own question, yes the are very worn. As they are less than £15 each, albeit the bike has 5 of them, I ordered a set. Picked up from Teasdales today after a pleasant ride across the Dales and lunch at Johnny Baghdads and fitted 15minutes after getting home. The wheel is now a really tight fit in them with less than 5mm rotational movement against 1st gear, compared to about 40mm before. Test ride after a cup of tea.
There’s 38,000 on it now, but it being one of those things that wears slowly over time they probably needed doing several thousand miles earlier. Symptoms were extraordinarily jerky throttle, unable to hold a steady slow speed, and a double bounce when shutting the throttle firmly. Put the bike on centre stand, in gear and rotate rear wheel against the gear. With the new Cush drives there’s hardly any movement compared to a load of slack previously.
I reckon it’s weight that does it. The bike is used mainly two up - over 200kg between us, plus all the crap stowed on the bike, then throw in a couple of loaded up trips a year and it’s hard work for the bike. All that heavy momentum has to go somewhere, a lot of which is into those small bits of rubber. I’ve also got through two sets of wheel bearings and a couple of spokes.