Thanks to Monster first and other subsequently.
I have been using my Evian spray to great effect.
I allow Diz one attempted snuggle under the chin (and the chomp has been replaced by a halfhearted graze) and then I get out the spray.

He disappears at high speed.
The good thing is that I doze off, and when I wake up again he is back snuggled under the duvet.
I know it's a simple response, but I might not have stuck it out if it wasn't for advice from here.
His little cat brain does not register rejection, he just knows that the spray is a Bad Cat sign and no longer tries to repeat it.

The Evian spray is perfect because it's designed to be sprayed onto a face anway, so I don't have to worry about hurting him or wetting my bedclothes. Also I can keep it within reach because it's an aerosol and won't leak.
I will look for a cheaper alternative when I run out, but at 1-2 squirts a night I might not even have to.

Cheers peeps.