LDV CONVOY LPG/PETROL ECU PROBLEM PLEASE HELP?

HI THERE I HAVE A LDV CONVOY LPG/PETROL DOHC, AND IT HAD A KNACKARD ECU UNIT SO I REPLACED THE UNIT AT A COST OF £700 THATS THE PART ONLY! BUT WHEN HE PLUGGED IT ALL IN IT WONT PROGRAM IN THE SIGMA ALARAM WILL NOT LET IT, AND WE HAVE TRIED TO BYPASS THE ALARM BUT THAT HASENT WORKED. PLEASE DOES ANY ONE NOW HOW TO DO THIS?? MANY THANKS CALLUM