Despite being as careful as I could in re-filling the master cylinder, I must have knocked a minute amount of crap from under the vehicle into the open m/cylinder.
Today, I changed over the master cylinder, bled the brakes and bingo! everything works fine.
On flushing the removed m/cylinder with de-natured alchohol into a clean wash tray, I noticed some very small amounts of crap floating about.
I suspect it was that which was being sucked into the holes in the bottom of the m/cylinder, partially blocking one or both, hence the time taken for the pressure to reduce and the switch to go off.
Lesson learned!
Be more than careful when scrabbling around under the vehicle through the master cylinder access hole with the top off the m/cylinder!!
Thanks for the help, Gents: all contributions to the discussion greatly appreciated.
Mike