Update : spoke to a neighbour (retired carpenter) who
used :-
1. cordless drill
2. 8mm De-Walt drill bit.
3. 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.