I've not heard of a swirl pot been used in this manner before. I'm more use to or associate the use of swirl pots in relation to fuel systems as they are used to stop fuel pump starvation and injectors running dry etc. Header tanks are a term I would normally associate with what you are talking about. Swirl pots tend not to have a point of filling which is what a header tank tends to have.
Have a look here for swirl pots:
http://www.fortunecity.com/silverstone/impala/1012/cars/tech/swirl/swirlfaq.html
What you are trying to replicate almost is your central heating system. You have a radiator, a pump, a thermostat, something to heat the water (the engine in this case) and some means of topping the system up, in this case the header tank is the point of entry. If you look in the engine bay of a modern eurobox, you'll see some form of plastic tank with a cap on which is the point you'll fill up. You'll also notice that it is the highest point in the cooling system this is important. From the plastic tank with the cap on the (header tank) you'll normally find to hoses, a big one and a little one. The bigger of the two is the point of entry of the water into the cooling system. The other is the pressure relief in the system, a bit like the escape to the road pipe that comes out of the top of the Mini rad that we feed into a plastic bottle so we don't loose to much coolant etc. In this case it cycles back into the header to be fed back into the system. The cap on the header tank is pressurised if the pressure gets too much then it will just vent to atmosphere with no means of catching it. So the reason why the height of the header tank is important is that if it is lower then the highest point then it is likely to try air in the system and result in locks and stop the flow of coolant around the system. The idea is that if the header is the highest point the air has a place to escape to. My experience in this is that all very well and good, but if you have a longish run of hose then it is wise to install some form of bleed valve on it to assist with getting rid of the air so you can get maximum efficiency from the system.
Hope you are still with me on this......!
If you want to keep with you Mini rad then you'll need a blanking cap as mentioned, a header tank, a t-piece and decide on which side of the pump you want the header tank to feed into. I think it is the inlet side of the pump and not the outlet (someone set me right here as I've not had to think about this in a while). It might be easier to do as Jason has done and ditch the Mini rad and get something that will fit the space you have rather then adapt around the Mini rad.
Here are some examples of header tanks that you can fit with the Mini system:
http://shop1.actinicexpress.co.uk/shops/Rat_Sport/index.php?cat=Radiator_Expansion_Tank___Swirl_Pot
Hope this all helps. Ask away if you have any more questions and if anyone disagrees with me by all means say so. I'm there to be shot down (it's my day job so I'm use to it).
Andy