[BrianB72]

I've read through a number of posts and very occasionally my expansion tank bubbles up, and I think its the cap.
I've ordered a new cap but does any one know the diameter of the pipe that attaches to it?!

If this fails then I'll be back asking questions about head gaskets and the like

 

[PaulR]

Good luck.

I had the same thought about mine but it was indeed the head gasket.

It is worth going to get it pressure/dye tested. It doesn't cost a lot and will give you a definitive answer.

 

[JRow]

I was hoping it was the cap on my spider being faulty with similar symptoms to yours,used a block tester kit (c£30 off ebay) in the end,and unfortunately,head gasket leak.
Hope you have better luck

 

[BrianB72]

The reason I'm doing the cap first is its happened twice and both times coolant has been coming out from the bottom of the cap and nothing through the hose, I had a similar problem on another car and the pressure release part of the cap was cream crackered, but if I took the cap off and pushed up the middle a handful of times it stopped it from happening!
For the sake of a fiver it's got to be the first place to go!