On closer inspection, the mount had sagged 5mm, and this explains the clonking I sometimes got on deceleration!
Cheap shite I didn't even think about when installing!
A little pressure added tightening using one finger on the vice handle squished it by 5mm
Above is where my gearbox sat on that mount
as you can see not much was needed to make contact.
After a bit of searching for poly mounts. I found one through Topspeed Automotive who managed to get one in in a couple of days! result!
You can just see, it rests nicely with enough clearance and as the mount is solid poly it doesn't compress anywhere near as much.