200tdi hot start problem

Yes you did read that correct hot start

Defender 200tdi starts lovely when cold -2 the other morning 5 sec of heat and starts on the first flick of the key every time!
But after a good run 20+ miles pull over to go in shop get fuel etc so out the truck for a few minutes! Jump back in turn the key and it’s like the battery is going flat , slow turn over and struggling to start !
This happen yesterday this morning again -1 and 3sec heats fire up first flick of the key again ???

Any ideas? I don’t think it is battery as cold start it cranks very fast ! Maybe starter motor ? But only when it’s properly hot maybe starter doesn’t like getting hot

You are probably right. A good check of wiring to the starter and earth straps is worth a try, but I'd say it's time to budget for a new starter motor