Thanks everyone for the input, it didn’t feel like fuel starvation but then it all happened in the blink of an eye, I did have a look up the ports and in the plug holes after I posted and it all looks good. It’s only done 1200 miles since it’s rebuild so it should be a fairly healthy engine.
What would cause the plug to do that, is it an impact or a fault within it?
I did hear of some counterfeit Ngk spark plugs being sold last year which made me wonder about these!

Thanks again everyone, much appreciated
Chris.