Many Thanks for all the help and information.
To update on my set up:
I am running an Arduino Uno with a Rogue Rmp3 shield attached and an Srf 04 sonic range-finder. The audio output from the Rmp3 is going into a little Kemo 12w audio amp, which is connected to a 8ohm/30 watt loudspeaker.
I need to power this lot for 10 days.
I have measured the Arduino/shield/sensor as needing 80mA 'at rest, and 110mA during playback ( the reading of the SD card is using this extra power I assume), this was tested with a 9v Battery plugged into the Arduino round power jack.
The audio amp is set at a low volume that is sufficient to playback speech at a reasonable level for listening at a 3m range. The amp/speaker are pulling 10mA 'at rest' and 80mA during playback. Surprisingly low, but the amp volume is way down.
I have worked this out over 10 days as requiring between 19.2AH and 26.4AH for the Arduino/shield/sensor, and between 2.4AH and 19.2AH for the amplifier.
I have purchased a Deep Cycle Leisure Battery which is 80AH capacity (so well over what I needed, but works out cheapest option compared to Sealed Lead Acid Deep Cycle Batts). I am also using a 12v solar charger.
I have heard that when fully charged these batteries can output over 14v, and possibly more with a solar panel attached, so to regulate the voltage I have run the power in to a L78S12CV 12v regulator. At this point I have taken a power line off to my Amplifier. However, before putting the power into the Arduino I have added a Pololu Step-Up/Step-Down Voltage Regulator S8V3A (as suggested by CrossRoads in another post I think..) to take the voltage down to an Arduino Friendly 8v.http://www.pololu.com/catalog/product/2120
Hopefully this is a more efficient method of getting from 14+v down to 8v, than simply using a fixed (little heat generating) regulator....
Leisure Battery turning up tomorrow, so we will see if the theory works in practise.
I offer all this information in order to hear any views, alarm bells, tips.. etc and to possibly help any other beginners some info on car/leisure battery use..
Apologies for the length of the post, and any comments gratefully appreciated.