White vapour/smoke from engine oil fill cap.

Driving a 1991 200tdi defender, had it a couple months and has been fine.

I have recent done a 200 mile trip in it and it had been running fine, however nearing the end I was getting a white smoke coming from under my bonnet, she still sounded and performed fine, maybe slightly rockier on idle but that may just be in my head. Popped the hood and there was white smoke coming from the oil filler cap. Does anyone know what could cause this? It may just be the seal going on the oil cap but I am unsure, have lost a little oil but have just topped it up for now. The smoke comes out just moments after starting her up again, even from cold.

Are you losing any water, and is the expansion tank blowing shortly after start-up with the cap off?

Do you have any yellow traces in the oil?

I haven't had the expansion tanks cap off yet because it's been running cool, bonnet and engine don't feel too hot even after I got off my long journey last night. I took the engine oil cap off the engine to top it up but there is no yellowy residue on the cap, just your everyday engine oil.

blocked breather or its worn out and breathing badly