I've never seen a factory fitting like that before, and yes, Nicads can and do leak. The solders on that look a bit too dodgy to be factory i would think. On my machine i went with a NVRAM, never have to replace batteries again, they wont hold time and date but all other settings and high scores remain in the system and you can then BLISSFULLY forget about changing batteries every 12 months

I'm just taking a guess, but I don't think its factory either. My BoP board was 3 double A (and your first pic looks like it was as well) and the soldering definitely looks like someone soldered that battery in there. I personally did the solderless remote battery from Marco and love how simple it was to put in.

The solderless remote adapter from Marco requires you to have a 3 battery holder on the board. So in this case, it would still require soldering. NVRAM kits are considered the best option on the market (that I know of) and at this point it would probably be best to go this route.

I personally don't know how long this battery would take to discharge, I am sure someone else on the forums will be able to answer that question for you though.

These boards had a charging circuit?!???!

They can be bought from a few places (all in the US that i'm aware of)
www.lockwhenlit.com
www.pinballlife.com
www.marcospecialties.com
to name 3, it's quite possible there is a distributor in the UK though.
I've bought spares from all 3 above and have found them all to be great to deal with, i would think the price will be fairly similar for all 3 also. Main difference may be postage charges!!