Aside from just Module 15, you'll have to get Module 11a. Your B1.2 license has Module 11b.
Differences that I can spot are;
Aerodynamics (High Speed Flight)
Air Conditioning (Pressurisation and Supply etc)
Electrical Power (AC Generation etc)
Equipment & Furnishings
Fuel Systems (Subtle differences)
Hydraulics (As above)
Ice/Rain Protection (Mainly Anti-Ice systems)
Lighting (To Level3)
Onboard Maintenance Systems (CMC etc)
So you've got most of whats required, just a few subtle differences to make up, not a lot really
Not sure where you stand with the converted license, a lot of the guys that converted from old turbine licenses now have restricted B1.1s, ie, no electrics/black boxes, so you may have the same problem. Best bet as its not that well documented that I can see is to drop an email to the CAA SRG.