Right it was not the big end that's gone, the problem wasn't the oil at all that was just a symptom of a much bigger problem...
The car already had a replacement engine which I wasn't aware of. Apparently the cam caps weren’t tightened properly (probably when someone took a look at or fitted engine) meaning cam shaft had a 3mm wide gap causing the engine to lose oil pressure and cylinder head to be damaged. It’s also the reason the car was burning oil so fast. The cam shaft is black from being burnt due to lack of oil, they’ll be metal flakes in the engine and the cylinder head will need replacing. The garage said the car’s been run like that for a lot longer than I’ve had it.
Unfortunately I just didn't have enough time to organise to take Blue_2.2's engine which would have been fantastic 🙁 so I'm still on the lookout for a replacement