Thanks for the advice, friends!
Bob, I'll chase up T&L but I'm hoping I'll be lucky enough to find someone closer to home - but who knows? I'll report on what happens.
Thanks Nev! I knew I could rely on you! I'm also wary of the heat transfer of sleeving but I fear it'll have to be done. On sidevalves, I've learned to follow Hartley's dictum of "run as rich as possible without loss of power" to stop things getting too hot. However, I fear the hottest conditions these days may be in stop-start traffic on motorways where you can't shut the engine off and don't have enough speed to give a proper cooling airflow. (You get a really hot right calf!) Probably not a condition I'm likely to meet these days. But regarding clearances, where am I with a piston that's just under three and a half inches across in a sleeve? Somewhere I've read from yourself and David Anderson that 5-6 thou was about right unsleeved but I can't find the reference and am not keen on relying on my memory.
To John, I've given the piston a good staring at and the "divots" above the pin are holes that go downward to the upper part of the gudgeon pin bearing each side in the piston, so it's got 3 holes for lubrication each side, two at the top and the usual one beneath. "Tadpoles" are the name of the inserts to make these holes, are they? Not heard that before! Anyway, the holes are nice and clear.
Thanks for all your help, MikeN.