I will get a helper and use the starter motor method I think, good call! Not looking forward to taking the spark plugs out to be honest - I took out 1 and it was very, very stiff (why do people tighten them like that??). I don’t fancy dealing with a snapped plug in this head, so plugs out will be last on my list :-O. My dad used to check the valves on his push rod powered Spitfire while it was running - I’m far too clumsy for that! Cheers! Mike.
You could add a wire from the battery around the ignition switch, but if you have a friend...
Don't know why they over tighten plugs. Used to cast iron heads I guess. I've not had one snap, but I picked up a head with no threads in one hole once. Had to have it helicoiled. No memory of what it cost. Someone who said he knew what he was talking about told me, for the Sprint, hand tight plus a 1/4 turn, though I've heard plus half and plus 2/3rds said as well. I stick with the plus 1/4 turn and never had one come loose.
There's at least one thread somewhere here on plug sockets - there are a number of issues with plug sockets for the Sprint, and it sounds like your lucky enough yours fits. I used to have two thin wall ones, one with a rubber insert for taking the plugs out and one without a rubber on a locking extension bar for putting them back. The one with the rubber left the rubber on the plug top if used to put them in, and the one without left the plug down the hole if used to take them out. Without the locking extension bar, sometimes the socket without the rubber stayed with the plug down the hole - a right proctalgia. I now have a very long, thin wall, magnetic, 3/8th drive one that cost me a tenner, IIRC, off eBay. That works an absolute treat, and I wouldn't dream of being without it anymore.
The OHV 1300 engine in my Herald that I do that with is actually 1/2 Spitfire - with the late MkIV big valve head on a Herald 13/60 block.