I have a similar problem, without using the car daily it only takes about 4-5 days before the car is a non-starter and she requires charging or a jump start.
I had the RAC sell me a "High Quality, High Amperage" battery, for which read " High Cost!" whilst stuck in the middle of Dartmoor on holiday last year.
I pointed out to the RAC tech that the car has a problem in that the drivers info panel LED's remain lit when the ignition is turned off (that's the outside temp/trip mileage and odometer bit). He measured the battery drain and declared that at .5amps it would be no problem and the battery was guaranteed to start the car for at least five years, yeah right!!
Does anybody have any suggestions as to how to test this, how to rectify this or should I just start practising for when I have to go all electric and just plug the car in every night?