After checking my cruise control system the only thing left was the ECU which had failed. I tried three used replacements that did not work. (My mechanic loaned me a ECU that was known to work & my cruise control then worked when I installed it).
The only way I found a good ECU was by asking the seller HOW they knew their ECU worked. The seller I found, (in England), tested it in his Disco 1 to confirm it really worked before he sent it to me. I installed it seven months ago & my cruise is still working.
I bought two of the three non working ECU's from ebay & returned them for refunds.
Good luck!