hi tom
did your car run ok before battery change ?
if so I would first disconnect and reconnect all fuses in both fuse boxes 1 by battery and 1 under osf of dash then check power going in and out with voltmeter to check all fuses are ok. (I had this once and it was because car had been standing ages and corrosion on a fuse under dash )

when you supply an earth does it start ? if so then you don't need to look any further just trace back the earth
if not and only the pump is running you will be just borewashing the cylinders and making it more difficult to start anyway

I would also make sure you have a spark as if not I would guess the immobiliser is the main culprit as most will cut spark and fuel so by disconnecting the battery completely you in effect turn it on

hopefully 1 of these will give a bit of help if not let us know what you find
mikei