Not my car, but trying to help out.
Car is a 2008 2.2D X type estate.
The car has had a long running rough/missing idle on cold start in cold conditions. In summer the problem is there, but much less so. Come the winter months and it's back at every cold start.
When hot the car drives and pulls well, no smoke at any time , no warning lights come on and the actual start is fine, just rough/missing and hunting idle.

I've used a friend's code reader on this a while back and got P100 code and P1664 fuel injection pump malfunction.

I've tried doing a code read today and got the same codes. With a 6 month gap between doing this, I was expecting the P100 to be gone, as the car has been in near daily use and covered 3k . Does that seem odd ?

I think a new pump is £500 ish plus fitting. Given the value of the car, not exactly something you want to spend quickly Ina ' chucking new parts at it ' exercise to try and fix it.

​​​​​​Would any more knowledgeable people go with replacing the pump or does the P100 code sticking not sound right ?
I can do basic servicing and repairs, but not owned or worked on any diesels before.

Thanks for any advice