Im prettty sure the alarm with ultrasonic sensors is standard fit.
Had it back apart tonight and finally found the cable tucked up round body brackets (!)
texted Previous owner and he was not aware of any issues..
Not sure whats gone on but maybe the first owner had it removed to avoid alarm draining the battery ? (car is low miles for age, so was stored for extended periods)
replaced sensor (heck why not, Ive bought the ebay part now)
no coding required
Used VCDS and cleared codes
Alarm led now flashes properly (not 10 seconds solid to indicate fault)
car been locked for 2 hrs now and still appears ok.
lets see what happens over the coming days, looks encouraging so far.
Next time you lock your car , check out the led flashing pattern, if it stays solid for 10-30secs you have some kind of alarm fault..