thanks guys, i managed to mount one new tweeter yesterday using a holesaw in the existing hole and some alcohol hand gel as lube, it was still a tight fit but the next hole up was quite big! I also got some converter plates to put circular speakers in the door.
So I take it that I ignore the old tweeter feed, use the old speaker feed into the cross over (small box!), then run from there. The cross over fits quite neatly under the old tweeter bracket so should stop things knocking around.
Hopefully will get some time to finish later on. I bought a Pioneer DAB head unit too (as i miss 6music!) 1/2 price at Halfords with built in parrot hands free etc but will do that another weekend.
So as a return bit of info, my tip for anything tight fit is alcohol hand gel! It works well as a lube for a short while, dries up quick when the alcohol evaporates and doesnt really leave any residue unlike washing up liquid or grease. Just remember it is flammable until it evaporates!