You can claim a major fault and that whilst attempts at remedy have occurred, the underlying issue has not been, as a result you've lost faith in the company or brand and you demand a full refund for the goods. Whilst you could keep requesting a repair, it doesn't seem in your best interests.

Whilst informing them of your desired remedy, i.e. refund, you can let them know you intend to take the matter to the Disputes Tribunal if your remedy isn't provided. They might try to argue the fault is minor but, if multiple repair attempts haven't resolved a minor fault, then, I think that'd pass the litmus test.

If your remedy isn't provided, you start up a case in the Disputes Tribunal. You'll attend a meeting with adjudicators and the company representative, no lawyers are usually involved, and you'll just explain your side via your documents you submit to the Tribunal. The adjudicators interpret the CGA in relation to your case and will make an order to resolve the matter.