P38 heater o-rings: draining tips?

Update: as we are in the process of moving home, I bypassed the matrix by cross-connecting the pipes in the engine compartment. People have said that they are ID 19mm, but mine are 22mm. Draining the matrix through the cut pipes was less dodgy than doing it in the car interior, and easy enough. I cut the valve out of a wheelbarrow inner tube, with a ~22mm disc of rubber. Removing the valve and blowing air using a tyre pump resulted in a steady flow of coolant. High tech storage in milk bottles with silicone Moo-pop caps. L-shaped 22mm x 100mm elbow linked the two pipes - refilled the cooling system in the RAVE approved manner (blowing through the rad bleed pipe) - job done. Matrix metal pipes blocked using wine corks (a decent claret seemed appropriate) - unnecessary but tidy. All set for fitting a Nissens 70230 matrix when I have time.