E53 Crank sensor location

[baconbuttyman]

My wife owns an E53.
For month now i have been tracking down the reason for a none starting issue,
It will fire up in a morning ok, run around for a few hours and then we stop, turn it off for a short time like to go into a shop etc, then it wont fire up again.
leave it for a while then it fires up.
I have replaced the following.

Relay in the e-box
relay on top of the fuse box (bit of a bugger to get to tho, glove box out etc)
Relay in the boot fuse box.
Fuel filter
higher fuel pump
Fuel pressure sensor.

I have had it into a BMW specialist which is not a dealership, in Blackpool (Auto-technique) and they were stumped, they guaranteed it was the fuel pump, £372 later, it didn't solve a thing.

We took it back to the specialist after it did it again, AA attended the car, they got it going with a squirt of bradex easy start, The specialist retraced his steps, he was the one today who replaced the relay in the e box, as they are a specialist, i am assuming they replaced the correct one. They had is nearly 3 days.
They are really scratching their heads on this

Wifey collected the car today under the instruction of "see how it goes", She stopped to get some meat from the butcher shop, low and behold it did not start, sprayed some easy start in the air intake, and it fired up no problems.

Is that a clue as the the cure to the issue, i know what easy start does and how it starts a car but, why is it curing the issue, what is is curing to start the car.

We have it booked into the main dealership in Preston for 2 hour diagnostics, but has any one any ideas as to what the issue could be after reading all this info

so i am looking at crank sensor replacement, where is it located