Hi Henry,
You will be familiar with tractor and machinery paint, well that is good enough for engines and it is coach paint, now called synthetic I think and if you can locate a local firm who mix car paints, they can normally also do these coach paints, Cromadex is one name. There was a company by Northiam station who I used to use, they have now gone. Worked out around £10-15 a litre at the time.
Try this company, I have bought from them;
M K P E, Capel Close, Leacon Rd, Ashford TN23 4GY
01233 612695
Category: Car Paint & Lacquer Manufacturers and Supply
By chance last week, I was in my friendly local engine machinist's workshop and he mentioned working on AEC cylinder head, putting two and two together, I think they were yours!
regards, Richard