when i got the new toy, the second key was and is like new, no marks or anything looks to have been kept in a safe somewhere

but used it today for the first time - and the dash has lit up, smart key battery low....any tips before i attack it with a screwdriver and break it!?

 2010 RRS HSE 3.0 tdv6 2010MY
2012 Audi a5 2.0 tdi s-line
2005 Discovery 3 tdv6
2009 audi a4 2.0tdi saloon
2007 Audi a3 1.9tdi sportback
2004 Navara Double Cab
1996 Shogun 3.0 v6
1984 Landrover 90 2.5d

The key fob has a rechargeable battery inside, it needs to be in the ignition to charge it. The car is telling you its low. 2006 RRS SC Bonatti Grey (the 55th car/van I have owned)
2005 GSX 1400 K5

mines a MY10 so keyless?? 2010 RRS HSE 3.0 tdv6 2010MY
2012 Audi a5 2.0 tdi s-line
2005 Discovery 3 tdv6
2009 audi a4 2.0tdi saloon
2007 Audi a3 1.9tdi sportback
2004 Navara Double Cab
1996 Shogun 3.0 v6
1984 Landrover 90 2.5d

It's in the handbook about changing the key, looks easy , just slides out. You get to it as you do the emergency key but it may be the other side.
From handbook;
When the battery needs renewing, there will be a significant decrease in the effective range of the Smart Key transmitter and the message SMART KEY BATTERY LOW is displayed in the message centre.
To renew the battery, follow the procedure below:
1. Open the side cover on the Smart Key by pressing and holding the release button.
2. Swing out the arm to expose the battery drawer.
3. Remove the battery drawer and remove the battery.
4. Fit a new battery, type CR2032 (available from your Dealer/Authorised Repairer), with the positive (+) upwards, in the battery drawer. Avoid touching the new battery, as moisture/oil from the fingers can reduce battery life and corrode the contacts.

Sorry, I did not know there were different ones  2006 RRS SC Bonatti Grey (the 55th car/van I have owned)
2005 GSX 1400 K5

A CR2032 is a standard battery - even supermarkets stock them so don't pay over the odds.