I am pleased to report that the drilling went well and was 100% successful. I only had to drill 6 holes altogether and 2 of those were only enlarging existing holes. I didn't break any of the drill bits and I think that the cutting oil
stopped the drill from getting too hot. There was generally nice long trains of swarf coming out of the hole, so all in all it was a satisfying process. Actually, the hardest job was enlarging the existing holes. I was probably only drilling out by 1mm (7mm to 5/16") so the drill had a tendency to "bite".
So now the new 6-sheave deck
organisers (Garhauer stainless steel
with aluminium sheaves) are all installed and looking very pretty. Thanks again for all your assistance, folks