Sympathies Dave - been down that road and faced similar decisions but currently all OK touch wood.
You actually have a couple of alternatives. Yes, you can get your cash back - there can be no argument about "not fit for purpose" - but if, as I understand it, you've already had the box repaired twice, then you could request a brand new replacement. Assuming you haven't tampered with it, your box is incontestably a basically faulty item.
Personally I would try this latter option. It's a shame to deprive yourself of so much pleasure just because you've got what is clearly a one-off rogue box.
You need to go back to the retail outlet you got it from, you need to show them proof of purchase and written details of your repairs. You should claim quoting Supply of Goods Act (below) not "Airnav Warranty" because unless you got a document with your box, there isn't one.
http://www.berr.gov.uk/whatwedo/consumers/fact-sheets/page38311.htmlHope this gives you some hope.