I've just changed from an 1150 to a 1200 and the handbook says that if you intend to charge the battery using anything but BM's own overpriced charger you need to disconnect it.
However my Optimate seems to work through the accessory socket - providing you:
1. Switch the igniton on.
2. Plug in the Optimate and switch it on.
3. Switch the ignition off.
The hazard warning switch then illuminates, which I think is a sign that everything's going well.
However when the battery is fully charged the electronic system switches off the accessory socket and the Optimate stops working, so you can't just plug it in and leave it over long periods.