I have a 2009 1.4 diesel Ford Fiesta. The other night I got a warning on the dash to change my key fob battery which I did.. the next morning car went as normal then after a short drive I went to start it up and it was like the battery was flat, just a clicking noise, I come back maybe two hours later and the cat starts again. This has been happening the past two days were I have to let the car settle before it will start again? .. I’ve checked the starter and it’s all good! The battery is reading 12.6 volts and 14+ when it’s started, I’ve changed the starter relay? Has anyone any idea what the problem is ?

Starter is probably knackered, resistance increases with heat so if the windings are getting weak that extra engine heat could be too much.

Alternative is battery cables for the same reason as above.

Years ago I had a problem like this. I changed starter motor which fixed it. Out of curiosity I dismantled old starter. There were loads of scrape marks on the armature outer and the inner surface of the outer magnets on the casing. It looked to me like the clearance was too small between between armature and the outer part and maybe when hot and expanded it  was jamming. It certainly looked like that.