There is such a thing as a bath rubber.I still have one that was given to me in 1984 from a sales rep.
It works on acrylic baths so might work for you ?
You will be changing the whole thing and the tail that screws into the rad will be1/2 inch BSP and the same as in your pic.
To remove the existing tail you might need a hexagon rad spanner or insert a wide chisel/screwdriver and undo using grips.
They will be 1/2 inch washers.There are two types,flat and domed.Most are flat.
Remove the flat cap on the top and you will find a screw underneath.After that you will need a spanner.