query re mercedes sprinter starting problem

We have an intermittent starting problem on our 2009 mercedes devon. Sometimes when I turn the key in the ignition , it fails to start. It still starts eventually, but can be after 10 or more turnings of the key. It seems particularly bad in damp weather. It recently had a service - on a fine day - and they couldn't find anything wrong. The battery is fine, they said starter motor is Ok. Has anyone else had similar experience and could they offer any tips please or shed light on any component that might be faulty? Thank you. Tony.

It might be worth replacing relays 7 & 8 in the compartment in the passenger side footwell. 7 is the engine control relay, 8 is listed as 15R, I don’t know what that means but I do know that is you take it out and turn on the ignition everything lights up as it should but the engine doesn’t turn over and that matched my symptoms when I had intermittent starting problems. The relays are MB part number A 0025422619.