In my radiator, the cap has a spring in it acting on a rubber seal, and above that is the overflow drain as normal.
This goes into a plastic bottle right at the bottom, the bottle has no other caps or holes or fittings.

How exactly does this system work? I'd have though any water that enters it wouldn't be able to get back past the radiator cap, and does that mean the only thing pulling coolant back into the radiator is the air pressure in the bottle?

I want to replace it with something more.......bling, preferably with a cap in it. Would that have any problems?

Thanks .

Directly under the cap is a rubber seal, this is where it differs from normal caps in old minis. When the radiator system cools and the steam pockets inside the block are reabsorbed, the radiator will now have a relatively negative pressure compared to outside the cooling system.
It will try and draw in air from outside to replace the water that was blown while it was running. The second rubber seal prevents this and instead it is drawn from the overflow bottle.

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

Okay, so the next question is, since I bought one of those chinese aluminium radiators, can I still get the system to work the same way?

I bought the chinese alu radiator and have the overflow connected to a plastic bottle. Works fine.

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1965 Morris Mini Deluxe - 998cc SC12

If there is an extra seal directly beneath the top cap then it is ready to go if a plastic tank is what you are after. I can't see why a pressurised expansion tank couldn't be used either....just doesn't need to be pressurised is all...

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.