I opened up the recessed connection box just to see what was in it. We have been in the house for 1.5 years, and the pool is 15 years old I believe. The light was working fine although we have not used it much, and only for < 30 mins at a time.
Lucky I did as the connection block looked like this:-
I removed it, and stripped back the insulation on the wiring, but it appears to have corroded. I suspect I need to replace the white cable from the connection box to the fitting.
Here is a pic of the light fitting itself. I have not tried to remove it as I do not know what is behind it. Can I remove it underwater, or do I need to drain the pool to below the level of the fitting?