Thanks for all your suggestions. The trapped air scenario described
our symptoms perfectly, so I tried dealing with that first.

The hot feed is taken from the side of the cylinder then vertically to
the attic, turns through an elbow and runs across the attic floor to
the pump mounted at floor level. There is a bleed valve on the elbow,
that yielded a little air but made no difference to the problem.

The hot outlet is mounted on top of the pump, from where a short
flexible connector picks up the copper tube at a point slightly higher
than the pump outlet. There is then a gradual pipe fall (10 in or so)
across the attic floor before dropping vertically to the shower mixer
valve.
There didn't seem to be a sufficient gradient to form an air pocket,
but I tried to bleed the system at that point by cracking open the
pump outlet connector with the inlet closed and the pump off. Much
water, but not possible to see if any air was released (I know this is
not sensible plumbing!). Trying the shower then produced some
coughing and hammering, but it all settled down & I ran it for five
minutes or so.
Since then 24 hours without any sign of the problem.

Not 100% sure of what I did, but if it happens again I can choose
between fitting an air vent just downstream of the pump hot outlet or
deactivating the hot flow sensor I guess.

After four years of use free of problems, let's hope for another
four.....

Many thanks for all your help.
Trevor
..