Is there any recommended stuff to coat the inner arch with to stop the terrible sound of stones hitting grp or atleast dampen it? I was thinking waxoyl but it is it too sticky and will just collect stones.
Cheers
Adam

what about some sticky backed neopreme or some other foam?

Rubberised roofing paint is the usual solution.

spread sikaflex type sealed on it

You can get cheap rubber floor mats from places like Wilkos, cut them up and glue them to the underside... Cheap and effective...

Stone Chip , made for the job . Available in aerosols or for a Shutz gun

Ear plugs....

CBS Wunderseal, i over did it and used 1 1/2 tins at £8 each.
Easy to paint on, you get high and it can be overpainted.