ok sorted now. The culprits were the rear lights. they are 10w rated and i had 5w in one of them. All the other bulbs in the rear clusters are the same - 21w so harder to get them wrong.
probably a common thing as then checked my 190 which have the same rear bulbs which should be 10w and both were 5w but as the 190 doesnt have a bulb warning light it never came up as a fault.
The system works like a Wheatstone Bridge - the car is divided into two halves and the bulb failure warning unit compares the current draw from one side of the car with the other. If the two sides are different/wrong the bulb fail is activated. This is why replacing bulbs in pairs tends to cure the problem
ok sorted now. The culprits were the rear lights. they are 10w rated and i had 5w in one of them. All the other bulbs in the rear clusters are the same - 21w so harder to get them wrong.
probably a common thing as then checked my 190 which have the same rear bulbs which should be 10w and both were 5w but as the 190 doesnt have a bulb warning light it never came up as a fault.