In reply to ebdon:
It's interesting that those Rambo ones don't fit G12s (although 20 year old G12s are a bit different to new ones). I've found in the past that you can swap heel clips around between different crampons and they still tend to work fine. I might have an old pair of DMM Terminator heel clips that you could try if you fancy. Are you somewhere Peak-ish? I'm Dronfield - if that's anywhere close to you, I can check if do have them and then either drop them off or you could come round and try them.
Do your G12s have the plastic toe baskets? That's the bit I'd be most worried about - my original ones snapped very easily. Got them replaced with newer ones by Grivel - they've lasted very well.