I think we interrupted Bridget's nap time last night. As EC summed up our lesson "It's not that she's unwilling or uninterested, she just has no energy to spare tonight." She's going to put in a call to the vet and pick her brain a little, because she's concerned that Bridget is so inconsistent day to day and so difficult to get fit. In the meantime, I'm going to try not to worry about it too much.
On the plus side, we learned Bridget can easily trot decent sized stuff on an angle, and I'd highly recommend this exercise as yet another deceptively simple pattern that's easy to set up yet difficult to ride well - you need to plan ahead and be very straight with super accurate turns off your outside aids.
All you need is a standard vertical, and 4 extra poles on a bounce in and out. The 'hard' way involved some super tight turns (about 8-10m) so it's an opportunity to let the exercise do the work on the adjustability of your canter as well.