Many years ago I retro-fitted Cruise Control, but I can't now find any details of the unit I fitted

It has worked perfectly until recently

Now it latches and operates fine for 30 seconds or so when first operated, then delatches

I can try again, but then it will delatch after 10 seconds

After a few more times it won't latch at all

The fact it does latch suggests that there is no problem with the speed sensor (or brake pedal test)

Any suggestions?

Faulty capacitors perhaps? Might try replacing any electrolytics on the control board just in case (haven't checked the board yet as it is tucked away and awkward to reach)