I was able, after a bit of a struggle, to remove the fitting from the ceiling and it bore a label stating "Ring-Lighting Art No. 50480 240 V Max 100W". While removing the fitting I handled the bulb housing, which is riveted in place, and the plastic/bakelite crumbled to my touch.
I searched in vain online to find a replacement then visited Litecraft at their Glasgow Showroom. There I was told that they stock nothing of this type of fitting nowadays.
I realise I am going to have to replace all four fittings, but not necessarily with spotlights as hitherto. So, I shall be left initially with four holes in the ceiling about 11 centimetres in diameter and am left scratching my head as to how this can be best achieved.
Possibilities which have crossed my mind are :-
a) To get discs of the same diameter cut from ceiling board and to use a strong adhesive to fill the holes, having first drilled them at their centre for the power wires.
b) To get wooden disks (also drilled in the centre) fixed with strong adhesive and marginally larger in diameter than the holes.
c) To fix plastic roses, though I have been unable to locate anything large enough for my purpose.
It would then be my intention to hang shaded lights or up-lighters.
I know from visiting this newsgroups for a long time that there is a lot of expertise to be tapped and I shall be grateful for any suggestions or advice anyone can offer.