range rover classic central locking malfunction

Hi , I dont know if any one can help me, ive just gotten my 1990 R/R vogue 3.9 back from the shop after having a lpg conversion fitted and, the central locking doesnt work any more !! Now, I know that this has nothing to do with fitting the lpg kit but its a bit odd isnt it ? has anyone got any ideas what the starting point would be to look for the fault (apart from fuses which ive tried ) oh by the way, i mean the key operated central locking not the remote type !
thanks in advance, Colin .