OK, so, I got the Wassell / repro wassell (I'm not sure which) stripped today. Modified the head steady mounts so it sits right on the frame backbone. Also removed the old tank mounts from the neck gusset, the 'cup parts were easy enough to remove, but they mount onto a raised section on the neck gussett. This turned out to be a pain as they are stamped into the gusset, which means that grinding them flat makes them so thin they may as well be removed, so, I ended up cutting them out completely, cut some plate to match and welded them in. Ground the weld back and now they're seamless.
Still got to weld some tabs onto the tank and fit some bungs to the frame so it can be mounted
Here's a few of the tank: