2.5 diesel '51 - starting problem after replacing battery

I have a '51 2.5 Diesel. I recently replaced a flat battery. Now the vehicle turns over but won't fire. Thought imobiliser problem - have tried key in door - no luck. After a bit of investigating on the 'net, noticed comments regarding 'BeCU going out of sync with ECU'. Could anyone tell me what the BeCU is, and where it is?!

its becm, and it is under the drivers seat, if its gone out of sync it will need pluging in to be resynced, or you can buy a resync box and do it yourself..

I agree it probably needs to be re sync'd. I had to get my 4.6 recovered to a specialist to be sorted. Charged me £80. Only £115 for the unit which you then have forever!