OK - answered my own (rather stupdid) question. As the above posts explain there are 3 bolts - a drain, level and fill bolt. I found I had to remove the whole undertray to get proper access to the drain and level bolts, but once you do it's pretty obvious where they are. For the fill bolt, as well as the battery tray you have to remove the air intake that goes to the air filter - very easy to do. You then have access to the fill bolt which sits on top of the gearbox.
I soaked all of the bolts with penatrating oil several times before attempting to remove them. For the fill and drain bolts you need a 3/8" square drain key. I was going to just use the end of a 3/8" extension bar, but if you do that the little sprung ball on the end of the bar means it won't go all the way into the recess of the bolt, and I was worried about possibly stripping it out, so I invested in a Lazer Tools drain key set from Screwfix and both bolts undid without any hassle. The level bolt was very easy to undo requiring just a 10mm socket and being very small. I got new washers for the drain and fill bolts from Honda but the level bolt doesn't actually have a washer so nothing to change there. The only other thing you need is a short lenght of tubing with a funel to refill. All in all fairly straight forward job - the most fiddly thing was probably removing the undertray.