Well after much toing and froing, it finally went into Crownhill VX on Monday after VX customer service agreed to 50% contribution.
Was quoted about £150 by the nice chap Vince on the Service counter. So dropped it off and left it hoping for the best.
Got a phone call late Monday to say it hadn't been finished but they had taken it off and the tensioner was snapped, thought it had sounded even worse on driving it down there. So he'd go back to VX and see if he could get them to up the contribution.
Came back to me Tuesday afternoon, all done sir, Vauxhall didn't give me anymore but I'll get the cost as far down as I can. All in £210.
What a difference, it doesn't sound like the same engine, it purrs on startup and idles almost silently. Wait and see if the fuel economy goes back up again, wife was down to about 250 on a full tank.
Full credit to Crownhill Vauxhall who after being presented with printouts of the field remedy and pictures of the parts that should be and the letters to and from other people they took up the case and got it sorted. Luckily before the engine went bang.
It cost me 210 quid but I've got a car I'm not worried about driving for the next 50k miles.
If you've got the same problem then push and push and push, with your dealer and Vauxhall customer care.
Big thanks to everyone on here who have helped out with information and links. Good luck to anyone else in the same situation.