BDMAlex has some good suggestions for you there
In all honesty I dont generally see warranties as any good anyway, unless its for obvious/serious failures, your issue falls into the 'could take lots of hrs/money to find' category. The way around it is to find an acknowledged specialist.
I doubt very much that your car has something wrong that will cause a serious failure.
Chances are the fault will change very little but car will run as normal
OR
whatever is causing the problem will just get worse, where it wont start/lots of ECU fault codes then it will be a lot more obvious whats wrong = it can be fixed straight away
Where are you in the UK? JKM are the kind of people who have found these kind of 'tricky' faults, so its worth a long drive.
In all honesty approach a specialist, discuss what the problem is/whats already been done, then agree a diagnostic cost.
(e.g 3 hrs labour to start then an update as to whether theyve found an issue or need more time)
(I wouldnt normally recommend it but you could also say 'what the hell' and get it remapped, any flaws sensors/fuel etc are going to be highlighted sooner
) But Id strongly recommend go to JKM get the fault fixed AND get the car remapped
Yes it may cost some of your cash (which in an ideal world you shouldnt have to spend) but then you can then enjoy the car