Springheeled jack.
Since the thread has decended into non arrival of Farnborough tickets (ours were raffled...I never won) here is what the "book" says
When an ATSU receives information that a captive balloon has broken free the appropriate ACC shall be telephoned without delay stating:
ƒ The type of balloon and whether carrying any person; ƒ Position of balloon site;
ƒ Direction and speed of drift;
ƒ Last observed height;
ƒ Length of cable attached to balloon;
ƒ Balloon operator’s name and telephone number.
There are then local instructions regarding action to be taken.
If the unit involved is SSR only a balloon will not appear on radar. A weather balloon MIGHT although I never ever saw the one launched daily from a site 9 miles south of me. Hope that helps.