Thanks for the reply, just changed it over.
To get the seat off I removed the 7 securing bolts that attached it to the movable carriage (13mm). This then gives you free access to the battery terminals and as you say brackets that hold the battery securely. Once all these are detached then the trick is getting the battery out. I did manage to eventually pull it straight up and out but when putting the new battery back in I found it easier to slide it in from the side, by removing the cover at the side of the seat base. Then obviously reconnect everything and pop the seat back on, beware the seat is easier to take off than pop back on, but eventually I got there.