Update, resetting the values by measuring to the arches seems to have stopped the bouncing around. Only had a couple of short sections at motorway height but I think it's cured it
I don't think it's spot on as the handling doesn't seem quite as good as it was but good enough for now.
I'll be making up some blocks when I get home to set it correctly. (30mm dia nylon is 27 quid a meter from rs components so I'll be ordering a couple of lengths of that)
Thanks again for all the help. This forum is fantastic!