Yes, the gearbox filler and drain plug, the engine block drain plug and later (11D) engine oil drain plugs are all nominally 21mm.
If you have not got a “normal” socket then a sparkplug socket is 21mm and will do the job admirably!!
If possible, in order to minimise the chance of damage to the plugs, I prefer to use a pure hexagonal socket rather than a 12-point jobby.
B…………….
Bernie