Thanks everyone.
I think I've figured it out now. I replaced my humalog vial this morning. The new vial has an imperceptably small volume of air inside it. Therefore, all the pressure applied by the plunger will result in the displacement of insulin through the needle in a continuous stream, due to the release of pressure. Sure enough, a nice strong stream when I primed the pen, and an insignificant droplet left behind afterwards.
The old vial has a significant air bubble within it. There are around 30 units of humalog still in there and at a rough guess, around a quarter of that volume is nothing but air. So, the pressure applied by the plunger will partly just be applying pressure to the air (more easily compressed) rather than the liquid. When the dose is delivered, the air pressure will be released, resulting in a lower actual dose of insulin, and a drop in pressure which will cause the insulin to drip, rather than stream. There will be insufficient pressure to deliver the full dose under the skin, so that when the pen is removed, this low-pressure insulin will slowly emerge as droplets rather than a stream.
At least this is going to explain my slightly higher than normal bg over the past 2 days. Always easier to blame the technology than blame myself. :shock:
All the best,
fergus