Can I re-use the old radiator tails/insert?
Radiator tips (probably no use for OP, but maybe for others):
When removing radiators, if it's on carpet I want to keep, or just don't fancy ******* up the floor, I drill a drainage hole in the bottom for a controlled emptying. Drill another near the top to allow for air to get in and drain faster.
To dispose, just leave it outside the house for the scrapmen to take.
Easy enough to fit a new one if the pipes line up