Hi Diana,
From my memories of around 25-30 years ago, the outer seals were renewable by workshops, but inner seals were part of base repair. However, I found a way of doing them and the vehicle examiners allowed me to do it. There are some special tools required to do the outer seals. I recollect having to stand the transmission on end to do the inners to keep things in place. If you have a sectioned drawing of the box then it will give you an idea of what is what. I do not have one to hand here and a lot of vehicles have passed through my hand since that time, so memory is a little vague at present.
regards, Richard