The separation of the hairpin wires is not perfect, but it gave me the ability to squeeze the loop with pliers to gently extend the hairpin length until it is about as close to 10.8mm as its likely to get.
In fact, remeasuring it just now, from side of vertical element to tip of hairpin is actually about 10.0mm. This is mostly due to the position of the vertical element. As the wire of the vertical element is 1.5mm diameter, just moving this about 45 degrees around the center pin will adjust the hairpin length to 10.8mm. Thats a job for tomorrow.
A quick knock up test, with the antenna clamped to a bracket on the side of the workshop, with a clear view of maybe 30 degrees of south western sky, and well into the evening when most aircraft are not flying, did show some ADS-B signals! They were not strong, and I lost them again after fiddling with the software, but they were there!