I had not usedmy 2003 A4 1.8T Quattro for sometime so the battery was flat - I attempted to jump start it from my VW Passat - system completely dead - no light from the control panel even when both engines wired. Leads are warm and I get sparks so, power is coming over - any ideas what is wrong? All fuses OK. Immobiliser problem or more serious?

The battery just might be shot. I think there is a point where they wont hold any charge at all. Try swapping the battery from the passat and see what happens.

Good idea! ^^^^^^^^^ That will tell you for sure if its the battery or something else.