Thanks for the feedback. I ordered a red one to see for myself, the cost isn't that high. And at least a LED should pretty much never burn out on you, so there's that. But if it sucks I'll just put the incandescent back in and go to plan B, whatever that is.
Looking at the Thunderbird Commander, the rear light looks pretty similar to the Tourings. £84 at WorldOfTriumph, but listed as not available...
http://indianautosblog.com/wp-conte...4-Triumph-Thunderbird-Commander-rear-view.jpg