Good news is I have managed to get the bike running after a 15 month rebuild!! But I have a question.. How much clearance is normal between the inlet valve on full lift and the spark plug electrode? I ask this as it wouldn't start today, so I took the plug out and there wasn't a gap!! Looking forward to your sage views.. 
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email (option): ian(at)integsci.com
It's an alloy head Ron and I'm using the correct N5C long reach plug. I've used a set screw with a nut (to adjust to the top of the head with valve fully open) in the plug hole and measured approximately 22mm which equates to the distance between plug washer and bottom of electrode.. It never rains but.. 
P.S. Just measured a N3C from my A65 and that looks like 21mm from washer to end of electrode, so I may have to be judicious on plug choice.. 
email (option): ian(at)integsci.com
It is possible the head and/or barrel has been skimmed and/or the plug seat has been spot faced at some point...
Also check that the inlet tappet clearance hasn't closed up...
An N5C plug shouldn't have any clearance problems with standard components...
The simple solution is to fit an additional washer of an appropriate thickness under the plug, either copper or aluminium...
The bike is looking nice by the way...Ian
email (option): ian@wright52.plus.com
Many thanks Ron and Ian for taking the time to help. I will be investing in copper washers..
email (option): ian(at)integsci.com
['this usually happens when an extended reach plug from the local car parts shop is recommended.']...???
A Champion N5C is the original plug specified for an M20/21 alloy head..Ian
email (option): ian@wright52.plus.com