The warranty is irrelevant.
Under the Consumer Rights Act 2015 -
"If you discover the fault within the first six months of having the product, it is presumed to have been there since the time you took ownership of it - unless the retailer can prove otherwise.
During this time, it's up to the retailer to prove that the fault wasn't there when you bought it - it's not up to you to prove that it was."
If bought within the last six months, go back to the dealer and request they repair the vehicle under The Consumer Rights Act 2015.
The only proviso may be surrounding the work you have had done, and whether this can be seen to have been a contributory factor towards the issues.