I hope I've solved it.

Last night I started working on Snowdrop. I adjusted the valve clearances, (tried to) put Redex down the bores, ditched the NGK spark plugs for Champion ones and fitted the downpipe-to-manifold gasket and it seems to have done the trick. I also adjusted the idling and the ignition timing which were both out. Today I've also bought some Holts Flexiwrap for the rear section of the exhaust as the exhaust bandage I'd fitted hadn't worked. Fitting it wasn't too bad and it has cured the problem. The exhaust is a lot quieter now! Snowdrop is running like a dream and since I did all the work she hasn't missed a beat. I'm thinking of going to the TSSC International at Stafford tomorrow and this would be an ideal long-distance road test for Snowdrop.