Riiiiiiiiiiight, update.
Bought a 2nd hand rocker assembly, didn't shell out for brand new parts as the casting is crap and the engine isn't really worth it. Flushed all the oil and coolant out of it, cleared all the coke from the pistons. Pistons and valves weren't badly seated or damaged so I left them as they were.
Put it all back together and it just wouldn't fire. Would turn over smoothly but no signs of life. Oh, and the starter solenoid would burn out if you turned the engine for more than a couple of seconds.
Bought a brand new solenoid, which arrived this Saturday. Ditted it, didn't work, starter wouldn't even turn. Re-fitted the one that sets on fire and double checked all the leads, then noticed the connection to the starter was lose and covered in crud. Cleaned that up and gave it a shot, it started!
Got it up to idle temp and then took it out for a test drive. 2/3rds of a mile later I pulled over to let some steam out and called Dad to bring out some water and coolant.
Not exactly sure what happened but I think the system was blocked or had an air pocket as when we poured more fluid into the tank there was a "clunk" from inside the engine and the tank drained straight away.
Left it overnight and topped it up this morning, sat it idling for a while and it got up to temperature and sat there. I used it to pop around town this afternoon and it kept temperature fine, not that it's all good news:
Oil leaks from the sump plug.
Water leaks from the radiator.
Fuel leaks from the fuel pump
Water leaks in through the quarter lights.
The electrics still prone to sporadic failure, especially the radio.
2nd gear is erratic in whether the synchro works.
The exhaust is a masterpiece of bodgery.
Bottom end of the engine does seem to have a bit of a rumble.
Also, rust spots all over the shop. I swear they weren't all there when I laid it up...
So, still a lot more work to go into it. Don't know how much longer this engine has left in it before it needs a rebuild or replacing...
Can't complain I guess, it's only cost me £150 or so to repair.