im finding that after a few hours use the power steering starts to get a bit stiffer, would changing the power steering fluid help

No.

there a post on D3 where by the steering column is lubricated at the side of the engine.
This may cause a short term fix and pin point the problem so a part (joint?) can be replaced,

https://www.disco3.co.uk/forum/topic164744.html


Not sure its an exact match with your issue

I had a similar issue on my last rrs. Eventually it got worse, then resulted in excess play.
It turned out to be the universal joint. They cost around £50 and isn't to major to swap. MY12 RRS HSE
Gone 4.2 supercharged RRS
Gone 90
Gone s2000

i did a lot of driving today, it seems to start getting a bit stiff after a few hours, when i turned the engine of for about an hour it was fine till about a couple of hours driving it started to give stiff again

Must be the heat from the engine near the joint.