Some pleb has filled my brother's SDi with blue g11 coolant. On the next service I plan to flush the system and replace it with the correct pink g12 coolant. I've just bought 2 1.5 litre bottles of g12 plus plus concentrate from tps. Does anyone have any tips for flushing? Right now my plan is to drain the system and flush through with copious amounts of water from the hosepipe. I've read horror stories about g11 and g12 reacting badly to produce a sludge that blocks the system.

I'd pull the bottom hose off the radiator and take the thermostat housing out and flush through with a hosepipe. That's what I did when I did a full flush.

As said just flush with hosepipe until it runs clear.