ok, she's all done ... sort of. It was as easy as everyone said.
Fits OK. Only problem is, as you can see, it's slightly tilted - rather than the centre run being level with the ground.
I had either choices: once twisted on, twist it back ever so slightly to get the right spot (it's still secure) - or - drill some new holes in the cap fitment. I tried that, but got the position of the holes wrong, which meant when put on, the cap didn't twist enough and was slightly tilted the other direction
haha
So I decided to leave it as originally fitted - and go for the twist on, then twist back ever so slightly!