Friend told me this story today.
Four girls are living in a agency houseshare, and recently the winter cold has started to bite, one who has what is undoubtedly an uninsulated roof loft extension also has a problem with the radiator not getting hot.
Rings agency to see what they can do about it, and they say it needs bleeding. "Fine, can you send someone out to do it then?", she says. "We won't send anyone out to do this, you have to do it yourself", they say. "But I don't know how", she says. "You need to get a tool to undo the valve at the end and let out the excess air" said the agency.
So after waiting weeks without proper heating and being unable to find one in the shops -it's the old style square socket apparently - she manages to find one on eBay by typing in "radiator key", and only 99p.
It arrives in the post on New Year's Eve, and later that evening, she lets all the air out of the radiator as per the instructions. Apparently it hadn't been done for a long time and there was quite a bit of air. Unfortunately it also reduced the system pressure and the boiler, as per design - refused to do anything.
Friend was called by one of the tenants as was obviously unable to restart the heating but was also unsuccessful and they had to call the agency emergency number the next day which resulted in a very expensive NY call out fee for a plumber to rectify. Apparently according to the plumber, bringing the system back online wasn't something they would have been able to do themselves even if they knew how as there was no filling loop in place.
I suppose in general terms, a tenant should be expected to bleed radiators if they are occupying a property for a long period, but only after they've been shown how for the first time - in that case, clueless tenants getting told to do that by the agent is just asking for trouble.