I have just swopped the expansion tank with the silicon bag in it for a new design one. The radiator doesn't have a drain plug in it so if I pulled the bottom rad hose off there would still be coolant in it. Would it be OK mixing the original coolant with some g12+ or should I flush as much out as possible with water from a hosepipe. The bag had not split thank goodness.

I'd be more worried about air pockets on refilling 

Coolant systems are often vacume filled i.e. get a vacuum in the empty system and then let let vacuum suck new coolant in, filling any high up air voids as part of the vacuum 'suck'.

You could end up with poor/no circulation if just pouring in liquid left an air pocket.

You'll be fine the vags seem to bleed very easily.

I have got the use of a vacuum bleeder if required, was just going to drop everything from bottom hose and refill with g12+ with vacuum bleed knowing there will be some original coolant in the system

You never get all the old stuff out anyway, maybe a few litres when system might take 7. Could refill with distilled water and flush to try get more out before adding the new stuff.

On 01/06/2024 at 17:55, Blue8793841 said:

You'll be fine the vags seem to bleed very easily.

naghh (slight squeak of anguish)....not necessarily.  I've seen some bad airlocks on Golfs/Audi's.  Remember that when they change coolant at the dealer, they do it under pressure to get rid of the airlocks.  It comes down to blocked/trapped air in the heater core.  You can let the expansion cap open to vent any air when the car is hot though (to be done extremely carefully and at a risk of scolding)

I usually just let the car warm up, heater in hot position, with the cap off untill the thermostat opens and then replace the cap.

Correct (and better that G13) coolant for topping-up is G12++ or G12evo.
It can be mixed with old G13

I didn't want to remove the bag in situ, just in case it split, so I just made sure that the engine was cold, sucked out the fluid with a turkey baster and put it into another container, released the expansion tank, disconnected the connector, released the upper hose and then released the lower hose with a pair of pliers.

Removed the silicon bag with a pair of long nose pliers and then refitted in reverse order, using the fluid I removed during removal. It saved all the hassle of having to top up an empty system.