Update : spoke to a neighbour (retired carpenter) who used :-
- cordless drill
- 8mm De-Walt drill bit.
- egg-cup full of thin oil (for dipping)
He managed to drill 10 holes in each 5mm thick mild-steel 'T' rail. He would drill for about 8 seconds, then dip the bit in the oil. It took about one minute per hole. I continued to drill the remaining rails using my B&D mains drill on a slow/medium speed.
The 8mm De-Walt bit has a (kind of) 3mm pilot head, so there's no need for pilot holes. Thanks for all your info.