clutch slip Defender 90 200tdi

I have a defender 90 200tdi It feels like the clutch is slipping when the weather is warm and you have been traveling for around 10 mins at the start if you step down a gear from 4th to 3rd it seems to pick up but then gradually get worse as the journey goes on and starts slipping all the time unless you only just touch the accelerator slightly. When its not so warm weather there is no sign on slipping and it drives fine . any ideas?

new clutch from what you say

friend had this recently and had new clutch and skikmed flywheel and still the same ended up being slave cylinder