close but no cigar...

double check the valve is the correct length 

 I figured it should be close to this measurement

 and so it proved to be close - but no cigar

Replace the valve seat

Fuck it, take it to an engineering shop and get the lot done. 

Then sanity finally reared its ugly head, the engine had been rebuilt with new valves and guides and cost is a significant factor and so dremeling the rocker cover for clearance will do till the next time the heads are worn out.