my audi A4 was starting as normal, then for no reason I could think of, it wouldn't. I changd the batter and it started as normal. Now even the dashboard lights don't come on'

Halo you mention that the battery were faulty,and you replace it.did you follow up why its getting flat,sometimes a battery can over charge and also goes flat and can blew bulbs,that can be a fault on your regulator on your alternator.

Yep this sounds like a classic case of bad alternator have it taken out and bring it in to be tested. depending on the age of it you may need to replace the brushes mine is a 1998 model and i recently had to replace the brushes due to them worn out. Be sure to check all your fuses as well as the alternator may have blown a few. there is a box under the hood and in the side of the dash. If you have a hand held computer diagnostic tool you can use this to check the cars voltage system. it should be running at 14 volts if its working correctly. anything lower and you have an issue as the same goes for anything over 15 volts

I've found what the problem was. The negative lead was not properly grounded to the car body, due to the lug screw being loose. The car starts fine now. So I guess the old battery did need replacing and in changing to the new one, I must have loosened the lug inadvertently. Thanks for the comments. I appreciate them.