Thanks for all the tips on rocker cover gaskets, guys. @Gordon Nichols - although there may be rocker cover stand-offs for bolts, the actual rocker covers are 'standard' clip-fit (with vent hole).
The silicone gaskets are £22 compared to the cork gaskets at £2. I'll go for the cork and grease combo..
@IaM-Ray she's a rescue Jack Russell with some Cairn/Westie mixed in to give those pointy ears and long hair. :-)