It's getting round to that time where one starts to think of giving the central heating its annual once-over.
The system here is a back-boiler vented type with TRVs on each of the six radiators, and will be 30 years old this year.
There are three 'down' legs from upstairs, there being no horizontal runs between the radiators downstairs.
Each of these - boiler and two rads - have a drain valve. These valves have a core, pictured here:
Are these still available? And from where? Do they have a name?
TIA