Thanks Jim, here in England it used to be an MOT failure if the battery isn't secured, I'm not sure if that is still the case but as you point out the battery can't go anywhere so I guess that is ok.

For me it was more a case of, I've spent a lot of money buying a new bike and expected the battery and tray to actually be attached to the bike :-)

I think putting velcro underneath the battery could be disastrous as velcro needs to be peeled apart and this would be impossible if stuck under the battery. I think I will just fix the tray to the frame and then put a strap over the battery if I get a chance.