Well, if I do it say it myself, this little adapter idea worked a treat. I’d used XT60 connectors on my motor and both batteries and one battery (which does not have an on/off switch) liked to spark when connected. By using an XT60 connected to the equivalent XT90s as an adapter, both on the motor and the battery, we seem to have very nicely solved the problem and that has saved me re-soldering the entire system. If you look carefully at the pics above you’ll see what I got and how I added heat shrink to waterproof it. Lubbly jubbly!
For anyone wishing to pick up a set of these themselves, I purchased them from this eBay seller.