What to do? I can't bear wasting fabric - especially lovely fabric. I wondered if I could break up the hexis with a second fabric. I decided to go with black and try to make a medallion quilt out of it. It was fun trying to reverse engineer the dimensions of the quilt so that the hexis would fit.
Anyway, the result is below. No longer a 'one block wonder', but now more 'stack n whack'. I am now working on a second attempt at a OBW, and I think this one is looking more promising.
What have I learned from this disaster? Fabric choice is everything. Fabric with lots of straight lines don't work - at least, not for me. You don't necessarily need a lot of different colours, but variations of tone and saturation are needed if I want to create the kind of effect I have in mind. Onward, ever onward!