Just an update.
I CAN CONFIRM THE ALARM UNIT LOCATION ON FACELIFT ML270 IS UNDER WIPER MOTOR.
To Access unit.
1) remove wiper blade arms (they may be very tight, if so stop them in direct up position, this will allow a removal tool to be used)
2) remove rubber trim along front of scuttle (in engine compartment) remove all three washer jets, remove six spring clips that retain scuttle.
remove scuttle, lift up left hand side, then fiddle off right hand side, it can be done but is tight.
3) remove two plastic nuts either end of under tray and remove plastic shield.
4) remove wiper assembly, 4 10mm nuts, two in middle, one at either end, disconnect wiring.
directly under the wiper motor is a plate with 4 10mm nuts, remove the two left hand side ones, and a 10mm bolt in scuttle panel, the alarm unit will now lift out of its hiding place, disconnect wiring.
I temp connected up me £15 second hand unit, result, worked as it should. Tomorrow i will extend wiring and fit unit in engine compartment, and again test. I will then put it all back together, but it needs a good clean, and i bit of paint on bonnet hinges first, i have also found another culprit that could be my water leak, the grommet that the wiper motor wiring goes through has lifted slightly, so will also seal that as well.
If i can, photo will follow at some point.