Key Batteries were checked and replaced showing 6.5 volts book says to ensure infra red window is clean, but no idea how to. Might try ultrasonic cleaner on Infra red window. But wait - latest attempt with the key released the steering lock but engine would not start for as with both my Mercs if it stands for a few days. The battery is knackered. So I swapped the battery for its big brother and with my higher capacity battery I managed to get the key working to the extent I dare risk driving to be harassed by the happy to see you, now we can sign you up and screw you merchants, lusting for their £50 diag fee.