Thanks all, all input is very much appreciated!
In particular d8veh - your posts are very informative and sounds like there may be a solution to the no key issue, thank you!
I've attached an image of the lock/battery area (not from the actual bike)... not sure if this is good/bad in a 'no key' scenario?!
I've no idea if the battery works or not, it's very cheap at the moment (it's the one you've linked to) so a bit of a gamble. It's sold as spares or repairs which may be a clue!