Finally had chance to pull the plastic boot tray to check out the rear wheel enclosure. Mudguard was fitted by dealer but it is clearly not sufficient to stop the crap getting flung high up inside the rear of the compartment. Anyway offending mudguard removed and modified; nice little plastic extension to the front using a cut section from an empty oil container which extends the guard down between the swinging arm. The original stops just short. The rear of the mudguard was tight close to the tyre so first I a put a small wood spacer between the tyre and the mudguard and carefully applied some heat to it from a heat gun. This allowed it sit further back and away from the tyre catching more spray. I then fashioned a nice extension using a closed cell foam sheet about A4 size that extends past the base of the rear end. Carefully mounted to the mudguard with plastic number plate screws and big penny washers. This now stops crap going up the back edge gap. Sorted. 😉
