Well this is interesting.
I now believe this indecent was not caused by the car being faulty, but by the dealers messing with something.
I have now found out that the dealer (Bristol Street Vauxhall, Keighley) is telling us bare faced lies.
They said that the car was returned to them (by the AA) with its oil filer cap missing.
I was there when the car was towed away and it was present and secure.
I have contacted the AA about this who have checked with the attendant who has provided written and signed confirmation that the oil filler cap was present and correct when dropped off.
It's now clear to me they are making stuff up to make it look like we have caused the fault with the car.
I've spoken to a couple of different Vx dealers who said that an insecure oil cap could cause the engine to burn oil by causing a leak into the manifold via the crankcase breather, which could cause it to take in oil mist and run rough, but not to the extent that oil should accumulate in the exhaust system. So the oil in the backbox is still unexplained
I think the dealers did something to it (not sure why when the car only went back in for electrical issues) and failed to put it back on properly, which cause the smoke & oil in exhaust incident the same afternoon we got the car back, and they are trying to make out like we have caused that by tampering with the car.
We got the car back yesterday and it smoked quite a bit initially but seems ok now.
However I am concerned about how this saga may have caused damage to the engine and catalytic converter. Should I have this checked somewhere?
My overall impression is that whenever there is a fault with the car, their approach is to deny that their is anything wrong, or if the fault is obvious, make out like the customer has caused the fault. They have threatened to charge us in future if the car is returned "without any apparent fault".
We have complained to Vauxhall customer service about this, and stated that nobody except Bristol Street have done any work on the car or touched anything under the bonnet, and on a 6 week old car we have no reason to even open the bonnet let alone touch the oil filler cap, but they are siding with dealers, saying that until the dealer accepts the car as faulty (obviously 4 AA callouts is not enough for them) we have no grounds to reject the car as faulty.
STAY AWAY FROM BRISTOL STREET VAUXHALL is all I can say, they are a bunch of lying con merchants!
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Its a CCRT 700