3.0TDI Hot Start Issue, Very Slow Cranking

[snapdragon]

On the way to get my headlights polished/restored today (which was a success), I stopped at the ATM for for the cash and when I got back in the car, the cranking was so slow but it started. Now, it has been doing this for about a year, not very often that I stop for such a short time, but when I do, it goes wuh wuh wuh wuh but starts, sometimes takes 2-3 times as the starter cuts out. The dash and DRL lights also dim a lot during this, much more than on a cold start, they nearly go completely off and somethings reset like bluetooth connections so voltage is is going too low.

Anyway a few minutes later, I was a bit ahead of schedule, so I parked up and checked emails and stuff, came to start car and wuh wuh try again wuh wuh.
Tried 20 times over say 10 minutes and by now was late for my appointment.
I was only 0.3 miles away so called him and he came out to jump start me and he checked the voltage on the battery it was 12.1v, tried to start it to see how far it dipped and it started normally without needing a jump start.

Went to my appointment for say an hour and started fine afterwards.

I'm sure it's not the battery, it seems to be the starter jamming (as the lights go so so dim and it cranks in a laboured slow way). Also, after this, it will start later on when cooled.

I am also thinking could it be earth straps? Can't say I have noticed any but I seem to recall A8chick having this issue about 5-6 years ago but can't find it.

This seems to be a commonish problem on these engines, but find lots of questions and not many answers, can anyone shed any light?

PS no codes about jammed starter.