Okay, I replaced the battery this morning without any problems.
Here are some notes to use in conjunction with any official instructions available if anybody else has to change one:
1) Remove rings, watches and anything metal from your hands and find some insulated tools.
2) Power down the UPS using the standby button and then remove all power leads from the back of the unit.
3) Wait a couple of minutes after removing the power leads and then continue to take the front plastic fascia off the unit which is just clipped on.
4) Remove the two screws at the bottom of the metal face plate. You'll need a slim socket set to remove the bolt in the top left corner.
5) The lead with the red connectors sticking out of the front of the UPS (no mention in official instructions) I left alone and didn't touch this. Later once you've removed the battery you'll find that this lead is only attached to the battery and doesn't actually connect to the UPS system. Interestingly my replacement battery from HP didn't even have this connection.
6) So now that you've removed the front metal face plate you have to slide the old battery out of unit. I found this impossible to do with the UPS system still in the rack. So remove the UPS system from the racking and the battery will come out much easier with a bit of tilting and gravity.
7) The new battery just slides back into place and is automatically connected. They look like about 10 small batteries that are all wired and held together inside a clear plastic surround which you must leave on.
Put the UPS back together again and re-mount on the rack. Once you've replaced all the safety covers you're ready to power back up.
Hope this helps somebody.