Ok so done a full load test just now.

Multimeter registered voltage which was +0.6V more than the OBC - I will state the OBC readings:-

Car switched Off: 12.5V
Car Idle: 13.5V
Car with full load (AC, lights, radio, HID fogs etc) - 13.0V
Increased revs to 2500-3000rpm under full load - 13.5V.

In my original post I had stated that when I switched on HID fogs from Idle the hazards and internal lights randomly came on. This time I switched on HID fogs whilst revving to 2500rpm and no issues.

None of the dash warning lights came on.

so do we think it's weakened battery? Will be visiting dealer for coolant fix and control arm bushes replacement so will have them health check the battery - service manager is a friend so I know he wont BS me !!