mercedes 311 cd1 02 wont start?

hi,my mercedes 311cdi 02 plate has been a good old girl so i decided to treat her to a new battery,disconnected old battery and fitted new one,put radio code and away we go or so i thought but no,dash lights up after a few seconds some lights go out but the heater plug light stays on also fuel filter light and edc light turn the key and nothing happens,have checked all fuses power is getting to starter but will not turn on key,what have i done wrong assuming mercedes have not made a van that is impossible to change the battery,the van has no posh bits like central locking and is just the basic model any help greatly appreciated as my hair is gettig thin,thanks peter.