Nice one Rob!
Bit of extra focal length never goes astray!
Another thing you could consider as a better option to a barlow would be using a Canon 1.4x tele-extender.. You'll end up with 1400mm as opposed to 2000mm with the 2x barlow, however it will better illuminate your sensor and snap straight onto the camera, and straight onto the T-ring.. The optical quality would be far superior to the 2x barlow too.. although, I guess its just one more thing to add to the ever growing "Things to buy" list..
Great work mate..