That sounds just like mine was, new set of valve stem seals, all sorted.

when the thurbo goes, normally you get smoke on overun, when you lift off the throttle.

You can fit it without using all of the elbow bolts,as the t20 has 4 bolts and the t25 has 5 it will work ok though provided you have an original nissan gasket.
I cant talk about the rover 1 as they are all extinct so I never see them any more,but the det 1 will work fine.

if it is the valve stem oil seals where do i get those seal will my local place that sells pistons,timing belts etc etc have them?

is it much of a job to do the valve stem oil seals with the head on?

how do i compress the valve springs to get the collets off?

If it is happening when the car is left idling for 5 minutes or so then it isn't the stem seals,it's the turbo.
If it's the stem seals then it will blow smoke whenever it has oil pressure,where as the turbo will blow smoke once the exhaust gets hot enough to burn the oil.
Engines can do weird things from time to time,but I have seen a whole lot of cars that have exactly the symptoms you describe in your first post that have knackered turbo seals.
I would say that you 100% need a new turbo tbh.

coolio irish i'll keep my lookout for a turbo then! i have contomplated getting a turbo rebuild kit but heard they are'nt great! or is that just myth from chat forums!

I've rebuilt turbos before with good success,but they are fiddly to do and you have to get them balanced afterwards.