FWIW at this late stage, I bought a MkIV DVD unit off ebay. I met up with the bloke who arrived in a brand new Mini. The DVD unit had a label displaying a manufacture date of 12/06 on it.

When I plugged it in, the display did indeed show the Mini splash screen and a slightly different colour scheme (all green and grey, eurgh!). I let it run for a couple of minutes, then turned the ingition off, then on again. This time, the BMW splash screen appeared with the correct blue and grey colour scheme.

In short, yes, it will fit and function properly - it will recode itself to the type of vehicle its being installed into.