Starting problem with 2013 Mercedes base Dev

From being a reliable starter. my Devon is now reluctant. Battery seems fine, and the engine turns over as normal. but instead of catching after 1 or 2 turns, it now takes 6-10. Makes me nervous to go away as planned next month. Any ideas please?

Assuming the starter is turning at the normal speed, I would suspect a faulty non-return valve on the fuel supply line. If turning slower, then the earth strap may be the cause.

Thanks....always nice to have an idea what's wrong before presenting it to a Mercedes garage.