Angus,
I replaced my rear main earlier. It is a little daunting but if the son of poor Newfie immigrants can do it so can you. You really need to get a proper drift of some kind. You will see different references for suitable items including ABS/ PVC pipe, etc.. The rub is getting sufficient (and not excessive force) to transmit through the drift to the seal all while trying to maintain an even vertical plane installation...if that makes sense. I actually drove mine a little to deep and had to fashion a tool to pull it out without damaging the seal. Let me know if you need info on that...I'd be happy to send a photo and explanation. Be patient...a whack at a time. Getting them started is a bit of a trick but once started it's not too bad.