My apologies for my somewhat misleading post earlier- it seems it wasn't the crankcase oil seal that I replaced when it leaked but the driven gear seal (the part the clutch drum bolts to).
I know this as I've just gone through my box of bits and have found a new crank case oil seal there as I bought the set before finding out which had leaked.
In my defence it was nineteen years ago so the memory has faded a bit but I definitely did it all in situ (clutch out obviously) and I don't remember it being too much of a bother to do that's for certain.
I also had a little look through my Haynes manual and it explains how to do all the associated bearings and seals so if you haven't already got one it's probably worth buying.
Later edition '91-'05 Variants (including the 620) here:
https://www.ebay.co.uk/itm/Ducati-60...jNzn:rk:1:pf:0
I have the earlier edition '91-'96 as that was the only one available at the time:
https://www.ebay.co.uk/itm/Haynes-Du...JfqY:rk:2:pf:0