Hi..Apoligies for the delay..
What exactly is wrong with the cap? is its stuck or just spinning on the tank?
Hi..there apoligies for the delay had a internet issue and couldnt get back on..
As per the cap they just un screw in an anti clockwise direction,sometimes you have to put a bit of pressure on the cap to undo - as its only a screw cap on these cars/expension tanks..by the sound of it as per you description - if its going tight when unscrewing then it sounds likes is crossed thread so it will only undo so far a retighten..maybe try a slight more force on the cap - failing that the cap "may" have to be cut down sides to release the cap from the neck of the tank..but as above try a little bit more turning force on the cap and see if that helps..I do hope this helps