Hello MacP
Welcome to the 21 owners club.
1) I am also looking into a new battery, and I don't want to modify the bettery tray I am looking for an alternative, will let you know the final result of my search later on. I work in Uxbrdige so not far from you.
2) To avoid air lock I run the engine a few minutes with the header cap tank off, it seems to do the trick on mine, and never had an air lock in the systems.
3) Bonnet catches, I have the part number somewhere if you cannot find it ion the list.
Jack
Emily, The Very Yellow 21