engine will not start after removing battery

hello everyone i require a bit of help i removed the battery from my freelander 1 2l diesel engine bay to carry out some welding repairs ,when the work was finished i replaced the battery and connected it up and my freelander would not start. The battery is fully charged so no problem there, the engine turns over without any issue but does not fire and you cannot hear the power going to the diesel pump and there is no heater plug coil coming on the dash when i turn on the ignition . i have a spare ECU, so i connected it up and turned the engine over and it fired up but only ran for 20 seconds or so then cut out. I have checked all fuses and relays and all seem to be in good order. ive spent a lot of time on here investigating freelander forums and have found some good advice of which i have tried but to no avail .I have also cleaned all the terminals i can find, but also to no avail. if anyone can offer any advice it would be much appreciated.