[Jim A]

Thanks for the replies.
When I mentioned recent familiarity, it was because it is easy to forget how nearly inaccessible the two bolts are. The only tool usable is a 3/8" wrench with the box end marginally the better.
One problem is that these bolts have had 60 years to weld themselves in place and another is that they seem to be 11/32", so the 3/8" (12 point) simply doesn't get a good grip. A 9.5mm six point might have been a choice
With a 3/8" six point wrench pushed firmly on the bolt head, I finally got one of them to round off. That so often feels at first like loosening!
So I think I have used up loosening options. Removal of the backing plate for a more brute force attack is tempting, but rhis requires getting the three big bolts (two castellated) off the steering structure, seriously buried in 60 years of grime.
The approach I think I will try today is drilling out the bolts from the front of the cylinder.
As noted, the bolts and the old cylinder are sacrificial.
One problem with old vehicles (though younger than I) is that 2 hour jobs so often aren't.

Prowbar: I am not sure what you mean by "brake line and nut."