you dont need to have it running just cranking for a leak off test, there are plenty of utube videos on how todo it plus a leak off kit on fliebay is about £35
there is a whole sequence of events that have to happen before the diesel will start (cant say about the petrol) any one of which would stop the starter from working
i have a 4.6lt thor which failed on emissions, apparently you have to drive a t least 46 miles for it to recalibrate if the battery’s been off, bit difficult when it has no mot tax or insurance