Third part warranty on BMW X5?

[AudiSport]

We have a 54 plate BMW X5 3.0d Sport. We've owned it for nearly two years now and bar the odd small issue it's been trouble free. It has now covered 65k miles and we intend to keep the car for another couple of years, however I've now started to worry about something major going wrong. I understand it has a weak gearbox and a friends X5 blew it's turbo at 70k mikes costing 1500 to replace..

So, should I start saving, or is there any extended warranties out there that are worth the paper they are written on? WarrantyWise and The AA spring to mind, I guess they are both going to want the best part of 80 pounds PCM? Which over two years will be approximately 2000 pounds. A lot of money - but quite a lot less than the cost of a new gearbox!

Any positive experiences with third party warranties?