on many cars you have to loosen the gearbox off a bit to allow the sump to come off, but you don't usually need to actually remove any of the gearbox bolts, just loosen them off enough to seperate them a half inch or so. not sure what the situation is on the TD but i doubt you'll need to do more than that. you may need to disconnect the nearside driveshaft from the gearbox to allow the gearbox to be moved away from the engine. thats a pig of a job as the nearside driveshaft is connected to the gearbox by 12 nuts and bolts that are a nightmare to get off. however you may not need to disconnect the driveshaft as there's usually enough play to seperate the gearbox from the engine with the driveshaft still connected.