My 96, A8 has been stood for 3 months whilst I've been away. I took it out to refuel and every time I took a corner water was coming out from the footwells under the dash. Any one got a clue what this is? its not the matrix is it?

 

It's your internal AC evap drains.

But also need to clean out rain drains under the hood..rain overflow can leak into AC housing, which is probably clogged and leak into the car, even without using the AC...happens to ALL of em eventually.

Pretty easy to take care of, unless you own NO tools.

Read this entire article and your problem WILL be solved:
audipages Unclogging the A8 Evaporator Drains