Thankyou for the replies.

Is there supposed to be a small circular piece of thin black rubber resting on top of the coolant neck where the screw cap fits onto? or is that small circular thin black rubber from inside the cap itself?

The reason why i am asking is cos as i remember that i took off the cap one day last year to top up fluid and i noticed the black rubber thing just resting ontop of the neck of the expansion tank where the cap would screw into.

Not sure what happened to it but i cant see anything inside my cap and theres just clear plastic on the neck of the expansion tank and i dont see a seal anywhere, not sure if that small black circular rubber was the seal and where it was supposed to be?

Ford have priced me £13 for a new part, my local parts shop priced me £6 so i went to look at that part but it looked nothing like my existing part so i left it there, and i did not notice any rubber seal inside the cap for £6...

Im not sure if the seal should be in the cap or attached to the expansion tank... and if it is supposed to be in the cap itself would i actually see it?

I am trying to figure out if i do need a new cap or not.

Thankyou