There is a Facebook group called Total 5’s that are linked to an mx5 specialist in Barry… if that’s any use to you. The Facebook group is mainly made up of people in South Wales.
Saying that, any good garage/mechanic should be be able to underseal for you. Also, there is a better product than wax oil. Dinitrol?? Perhaps someone else can answer that.
Ah funny enough I’ve been to Mark in Total5’s a few times, hes fitted springs and a few other bits for me… had no idea he did undersealling too! I’ll pop him a PM maybe after xmas… Cheers folks