Takes about 20mins and is well described in the Haynes manual.
You'll need a trolley jack and an axle stand, although you can get away with the supplied jack if you have a couple of wind up type axle stands (you need to remove the nearside wheel, place an axle stand in the usual place to support the car, then use a jack/wind-up axle stand to support the gearbox -- use a peice of wood beween gearbox and jack, etc).
Given that it's so simple (but requires maybe £25 quid worth of kit which you may not have) it might be worthwhile getting a dealer to do it.
Are you sure that only the gearbox mount is knackered?