Job done today. No air locks! I just went for the rad drain plug and not the cylinder head one as well. Once all drained I stuck a hose pipe in the expansion tank to flush anything else through and upped the pressure and fed hose directly into rad. On the first refill you can squeeze all the hoses to get air out. It tales a minute or so of the engine on until the level goes back down again then you need to make sure you have enough coolant mixture made up to top up with - that caught me out at first and I had to switch off and start up again. Top hose hot. Switch off, cool down and re-check. All good
I don't know if there is a suitable size nut to fit the drain plug but an adjustable wrench round the outside was good enough.