ok makes sense Matt, I was thinking of maybe heating everything and then squirting the bolt with that "cold stuff in a can" that someone posted a link to on page 1.
Andy, thanks for the tips. Really not sure about heli-coiling or re-tapping up to M8 as I can't see how much material I have to play with until the plate is off.
Plenty of advice for me to go on here, I just need to decide which approach to go with...
1. Do I leave the bottom bolts (and so the plate) intact and try and drill/"easy out" the broken studs (with heat and plusgas too) and replace with new bolts without ever removing the plate?
2. Do I attempted to remove the bottom bolts by using heat/plusgas/luck/power of the jedi but risk snapping those off too and thus ruling out option 1?
Having just looked at the engine, I'm not sure I will be able to take the block off as after initial inspection it seems I may have to remove the rear coolant ducting plate which may well lead to the same problem!