I think I bought the wrong product - Ordering in haste I went for Polyurethane Liquid Plastic Casting Resin rather than rubber.
I have actually now bought a tube of Sikaflex 252 which seems to be popular for this kind of job. Many people on YouTube using it for filling engine mounts to stiffen them up. I'll give it a go. The dogbone bushes look a little worse for wear too so I may try it on that too.