Re: Cutting out
to clean the throttle body you need to remove it then clean out the inside with your preferred cleaner; WD40, contact clean, brake fluid, carb cleaner, whatever you prefer they all do the job nicely. also disocnnect the battery while you do all this, then when you reconnect the battery the ecu will relearn the idle and throttle ranges. within 100 miles it should have a stable idle at 850rpm.
its still possible (likely) that the stalling is due to the crank sensor, and i imagine that is what they will replace under warranty (with a bit of luck, with nationwide you really need some luck). when you get it back if the idle is still erratic you need to clean the throttle body.
nationwide warnings have appeared on watchdog and MSE and i think they were the garage who ripped off the granny on that rogue traders style programme in 99. main complaints are unneccessary work been sold to people and work that is paid for not getting done. in your case this isnt a problem since you arent paying.