I have had these occasionally come in as non start.
First one I had though had come in for an MOT test, after the test it was left in the workshop while we got on with other things, came to move it about an hour later and it wouldn't start. It had lost some other functions such as rear wiper and central locking was not working and immob light was on.

After trying several things I removed the UCH to check for water/damp ingress, it all looked OK.

Refitted the UCH and everything was fine. And the customer has had no further problems with it.

Probably had it two or three times over the last few years now - I have never got to the bottom of, but it seems to rectify the problem and have not had any further issues with the car since.

And they all seem to be about this age of Clio as well.