The part is only about £25......I changed mine at the side of the road in around an hour - its just about possible by removing the battery and the inlet trunking, and reaching down the back of the engine.
The sensor is held in by an Allen screw, and you have to manage by feel to get the Allen key in, and in my case I used a cranked Allen key with a spanner on the short end to give me the leverage to undo it. Once the screw is out a certain amount of persuasion is need to get the sensor turning back and forth before it will free off enough to come out of the block. The wiring to the sensor is clipped to various points on the brackets on the back of the engine and the inlet plenum...they are a pain to release so I just left the old sensor dangling...
Once you've found the hole in the block that you've just removed the sensor from - harder than you'd think (!) the new sensor just pushes in and you wiggle it about until you can get the allen screw back in. Reconnect the new sensor to the plug on the plenum chamber - there are four plugs on there but they are colour coded so you can't get it wrong - IIRC the crank sensor is a brown one.
Job nearly done - so flushed with success you try to start the car to check it........then you get very embarrassed and go back under the bonner to reconnect the inlet ducting and, rather more importantly, reconnect the battery
Now the job is done.....and hopefully all is well.
I hope this helps...if it doesn't, feel free to give me a call on 07976 384980 and I'll talk you through any issues...
E&OE - I don't guarantee that this will fix your problem....it did mine, but I'm only surmising what your issue is.
But bottom line is if I can do it by the roadside in an hour, a proper garage shouldn't take (and charge for) more than a couple of hours