One of the nice things you can get from CTEK is the comfort indicator, it has colour-coded (red/amber/green) LEDs which flash about once per second to show you the state of the battery and lets you know if it could do with charging. If the battery needs a charge, the charging lead on CTEK chargers plugs straight into the comfort indicator - although you could use a different make of charger and charge via the under-bonnet points in the normal way
Mine is in the boot permanently connected to the battery via the eyelets and threaded up and into the boot space so that I can see the indicator lights whenever I open the boot. It should work just as happily connected to the under-bonnet charging points and, if you're storing your car for the winter you should be able to arrange the lead so that it pokes out of the car at a suitable point and can be seen when the car is locked - leads are a little short for this but some additional cable and heat-shrink tubing should sort that out.
Costs under £7 (delivered) from Amazon http://www.amazon.co.uk/gp/product/B002MT8S7E
They also make a panel socket version which may suit the M cars as it looks like it would fit nicely on the wall of the 'battery box' (assuming there's enough clearance) and create a plug-in charging point in the boot http://www.amazon.co.uk/Ctek-CTE-56380-Comfort-Indicator-Adaptor/dp/B002OHXX60