try the classic car people,
starting at the A's I found these guys who are incidentally only 3 miles from where i live, i know they advertise in eBay
ABC Spares - New spares for all vehicles 1925-1975. Specialists in ignition, plugs & service parts. Lucas, Delco Remy and Champion
32 Hythe End Road,
Wraysbury,
Staines,
MIddlesex,
TW19 5AR,
Great Britain,
Tel: 01784-481712
Email: richardjnorton@hotmail.com
failing that there are plenty of spark plug suppliers who attend the classic car authojumbles especially at Beaulieu if you can get there