Hi Joh
I run a VX 2.0 injection and had similar problems when useing the flat Rover expansion tank. Always leaked from th cap after a good run.
Just changed to a lager round VX tank from a Cavalier and mounted it drivers side or cross section with new brackets.
Larger capacity, better preassure cap, and mounted higher (fluid level now above top of rad)
Worked so far evn after a 100 mile run at high revs.
Regards John