Td5 problems

hello there, new to landyzone!!!
I have a big problem, I have a discovery td5 reg 2000 and the other day battery went flat so bought a new battery and thought problem was solved. next day got in battery was flat so put on charge and started fine, got in again and nothing dead as a dodo. but now new problem occurred, turn ignition on with battery reading 13 volts and no ignition lights, turn the side lights on and the dash lights up, so scratching my head abit then problem goes on to now turn ignition on and then side lights so dash lights up but fuel pump dosnt run and you cannot start at all, as soon as you go to start all the dash lights go off and when you return key to position 1 lights come back on. I have read a few places that it might be fusebox so I have put a new one on and no change. just need a bit of guidance and help on this problem.

Try the relays (yellow) next to the ECU, pull them and give them a clean. Sometimes the connections in the socket aren't great and quick waddle sorts them out. Also clean the earth that's near there.

Sounds to me like your alternator may have gone open circuit. I would recharge your battery then do a voltage drop test on the alternator.

Check the tightness of your terminals on the battery, they have to be really tight.