Just spoke to Matthew at Giallo.
The problem was the cat on the drivers side has completely been destroyed, and it's blocked the exhaust.
The fix. New Larini cat's!
So they will be going on when they come in, which will be a week or two.
Apparently I was fortunate that reving the engine as we were at Brooklands, blows the **** backwards when they're failing so it reduces the chance of any being sucked back.
Which can happen if you just switch off the engine after a drive.
In summary it will cost about £2,500 which is a fair bit cheaper than a new engine.
And she'll also go like a stink. And be a lot louder.