Dell XPS 13 with persistent bluetooth problem

I've had this XPS13 9380 laptop for about a year and half. It's on extended warranty. It's generally very nice but has a persistent issue with the bluetooth chip turning off. Bluetooth is pretty central to my working methods. It should just work. I spent a lot of time researching fixes on line. It seems this is a common problem. I experimented with several fixes over an extended period, to no avail.

Dell replaced the motherboard (bluetooth is on the motherboard). This did not solve the bluetooth problem. The screen became defective shortly afterwards. The second motherboard and original screen were then replaced. The screen was then OK but the bluetooth problem quickly manifested again. The old fixes found on-line were as ineffective as before.

Dell advised me to reinstall the OS as a last resort before they would replace the machine. I did this, full reinstall from a fresh copy of Windows from Dell, updated all drivers and the BIOS.

The machine worked flawlessly for ten days or so, then the bluetooth problem began again. I reported this immediately to Dell, only to be told too long had elapsed between Dell's instruction to reinstall the OS and reporting the recurrence of the bluetooth problem and they would not honour the replacement offer. This was pretty hard to hear. I had not been told there was a time limit and Dell knew the problem took time to manifest, because I had explained this on several occasions.

The proposal now is to restart the entire process by replacing the motherboard again (this will be #4), in order that the case can eventually be re-escalated to the appropriate level of support for a replacement process to once again be considered. They're having a laugh right? From my side it feels like Dell is using "internal processes" to avoid replacing a laptop that Dell is unable to fix. This has now being going on since August.

To date I have spent 20 hours researching and applying bluetooth error fixes found on the internet and 18 hours reinstalling windows and the 18 pieces of commercial software I need to do my job. I am self employed. This is costing me a lot of money. I'm doing Dell's job for them. I don't work for Dell!

I have to say, that while Dell's staff have always been courteous and helpful, Dell's attitude to this case as a corporation is costing me a lot of money and I am extremely unhappy.

I've tried to escalate this via the Dell complaints website. What else can I do to get redress?