The small internal threaded piece that screws into the centre of the twin feed valve has come out, and I don't know how I can get it back in. Also the black plastic pipe from the radiator side has come away from the valve.
What is the easiest way to reassemble this valve?

Or where can I get a replacement ?

Turn off radiator, bleed, replace valve and fittings as needed. Piece of cake. The hardest part is how far away from Home Depot or any other plumbing supply store you are and if you get a good parking place.

Your old parts may be stripped so why bother with reassembly but you can try.

Hi Guys

Home Depot isn't going to carry this Twin valve and without seeing the exact valve I can't tell you how to reassemble the parts. Your best bet is going to be to bring a picture of the twin valve and the black plastic pipe to a local plumbing supply wholesaler and see what they can figure out for you here. Most likely they will be able to get you parts that are pretty easily exchanged.

Mark