I own a 2006 BMW X3 2.0d Sport, which at 53,000 miles has suddenly lost all power. At the time it lost power, the car was towing a car and caravan.

I took the car to a BMW main service centre and ask them to take a look at the loss of power and also a brake issue (loss of braking power on repetitive use). BMW advise £100 give or take for diagnostics to which I agree.

Three days later I had a call saying the EGR was not problem and they needed another three more hours to find problem which would take the bill up to £700. They did state if they found the problem and could fix it in the £700 then they would do so.
A week after that I had a call saying they had diagnosed the issue and the engine needed a decoke, this would take the bill to £3800. After a long discussion I said I would think about it over the weekend. To me I did not think it was decoke as it was not a progressive problem, more of a sudden loss of power.

I paid the £700 and took my car to an independent garage where we discussed the issue, I gave them the single sheet report I got from BMW and left it there. Brakes not fixed also.
Quite quickly the garage said whilst there is some coke, nothing near the amount to cause this problem. He said it is the turbo, ok so £1900 to fix all in.

I picked up the car tonight, he showed me the turbo, the "banjo pipe" which was clogged, a simple case of oil starvation breaking the turbo. They can’t believe BMW could misdiagnose such a simple and common problem and in theory it would of cost me £6000 to get the car back on the road

So the question is, I’m seriously annoyed at BMW and its clear the £700 diagnostics was a rip off, where do I stand legally to get the £700 back?