Evening gents, thanks for the replies,
James the oil cap has a very slight vacuum on it if you like but it's not hard to get off like the vauxhall type fault.
I've checked out the PCV and it's all working correctly and I found some info that confirms that these will run rougher with the oil cap off.

Alan your quite correct it's after it has warmed up about 10/15 mins later it will start to misfire. I was starting to think the same thing regarding the rings stuck as the oil quality that I drained out was appalling.
I should ve really thought about this first !!!

Thanks Russell that's pretty much what I've explained to them but I told them that because of the low compression and the head fault that there was no way of me really finding that fault until it was 100% gas tight and if there was no fault found with the head then I would've looked to the rings anyway.

Does this happen to many of the 3cyl engines as I've had a corsa 3cyl that's pretty much bit me on the arse like this one has. It doesn't half make you question yourself sometimes.

I shall be in future making customers aware of potential problems !!! Another learning curve for me anyway !!

I wouldn't mind but I did this job dirt cheap as well trying to help her out !

I think she's going to trade it in anyway now. I'll keep you all posted !

Thanks again for the advice