I came out to my car yesterday and it was completely dead. Tried jump starting it and got it going but it died again after about 20 seconds. Got it jump started again and kept the revs up to try and keep it going in order to drive it and get some charge in the battery but it died again at the end of the drive. Got it back on the drive and jumped it again and managed to have it running for about an hour at idle, while it was running I tested the battery and alternator, they were both running at about 14.2 volts so seem fine.  Then went out again an hour started it up and drove it but it died again down the road. Before it died the brake symbol flashed in the centre of dash and beeped a few times, after looking this up online it seemed the battery was at low voltage and triggers this warning by mistake.  This morning I go and buy a new battery, replace the old one and now theres no power to the car at all.  

I'm at a loose end and have no idea what the problem is so any help is greatly appreciated!

Haven't got a dodgy earth cable have you? To me, the overall issue being worse after replacing the battery would suggest a connection issue.

Ian