Ok, So this morning I went to maplin to purchase a cigarette lighter power supply to drive the T20's, The young man in the store said he had nothing that would suit my application and advised me to go to Halfords across the road as they deal with car stuff.
In Halfords I purchased a multi-voltage, multi-device charger.
This thing plugs into a cigarette lighter socket and can be set to 3, 4.5, 6, 9 or 12 volts and can be set to positive or negative center pole.
The T20 were positioned in the 2 front storage compartments using double sided reg. plate tape.
The charger was plugged into the 12 volt ac on the back of the right speaker, the left speaker plugs into the back of the right speaker and the 3.5mm audio in was plugged into the right speaker and then into the audio out on my Creative Zen Touch 20Mb.
Power on the charger, power in the T20's, power on the Zen, and listen to the sweet sound of all my Mp3's at full tilt all day long.
The 12volt dc from the charger goes into the 12volt ac of the T20's, through the bridge rectifier and powers up the T20's without fault.
On this occasion, 12 volt dc to 12 volt ac needs no extra equipment.