Just to conclude this thread. The car had behaved itself since my last post and as it didn't show any faults on Star, it was left alone......and then it did it again last week!
Decided as I didn't want my elderly parents being stranded anywhere, contacted MB for a new starter and it was circa £250! Quick bit of googling with the part number and found a new Bosch unit for £60 from a reputable individual on the bay dreams (he had purchased it and never used it), so along with some MB oil & filter it went in for a service, MOT, and fitment of the motor.
Pig of a job apparently involving dropping of the front subframe to allow access (easiest way to do it) but all done now and passed its MOT, albeit an advisory about rear brake pipe being corroded!
For ref, Bosch part number was 0986018570, to fit a 2008 W169 A150 Auto (petrol)
Fingers crossed, it is all resolved now, as always, thanks for your help folks!