What a miraculous result - ended up using RTV to seal plate back, and a bit back on the head for the rocker cover. The results are amazing - not one bit of kangarooing - much smoother. Cheers for the tip.
BTW the reason the astra don't do it is cause its got a standard exhaust I think.
Also thanks to Mike who's talk thru and diagram on archive was very useful.
I don't want to rain on your parade, but I would wait until you've driven the car around for a while, before you start to celebrate. I have had kangerooing for over 14 months and I have changed, cleaned and swaped everything, once twice and more, now I havew given up. I have changed the idle control valve 5 times, changed the air flow meter 3 times, changed the oil filter in the cam cover twice and added, changed and cleaned breather pipes till I'm blue in the face. Everytime I do something new to the engine, its fine for the at least 25 miles and I think I've cracked it but it still comes back, so I've given up the ghost. So now I'm just leanring to live with it. Until its time to get another car. Anything but a vauxhall.