Well I think it's sorted.
I went back to the Lemforder mounts I was using that previously gave a more muffled knock than the harder polyurethane mounts.
I ground about 1mm of the inner metal sleeve on the bottom side.
I also added an extra rubber washer to the top convex washer.
The result of this is when torqued up and the metal sleeve is clamped between the upper and lower convex washers, the surrounding rubber (with knobbly bits) is clamped up more firmly, between the convex washers.
All seems quiet now.
Thanks everyone for your thoughts/suggestions.
