I suppose there is an airlock somewhere in the system, probably in the
boiler, so would it be safe to connect a hose between a mains pressure tap
and the drain cock and blast mains pressure water in reverse through the
system until it appears in the header tank?

Yes, do this. One of the reasons I prefer sealed pressurised operation is
that the standard filling loop does just this, making refilling a doddle.
Check the security of the overflow first, though!

If you find where the airlock is likely to be, insert a bleed valve there.
Look for large inverted U-bends.

Christian.