Dave, Have you run a check via the OBD2 connector to see what codes the car may be showing. An OBD2 code reader is a very good investment and need not be extremely dear to be beneficial. It may prove a financial advantage to try and diagnose what is /may be faulty, rather than just keep throwing money / time at numerous items in the hope of finding/eliminating the problem . I have a Foseal Reader which are fairly cheap and I manage my S-Type quite successfully with kit.
https://www.ebay.co.uk/itm/363255573487?epid=20043258787&hash=item5493b82bef:g:AT8AAOSwlZFf~TIY
Good Luck with your diagnosis and correcting the car.
Best Wishes and Regards, John