Thanks all for your replies. This happens maybe once a month, usually if I've under-bolused so goes up to say 12, but then it just keeps on going rather than coming back down like it normally would with a corrective bolus.
The tunnelling sounds interesting though, because often it happens when I might say be going out for a couple of drinks or some food and I continue to eat/drink (not excessively) having thought I'd bolused the correct amount to a) bring it down and b) for whatever I might eat. So I might give myself 6 units - 2 to bring it down to the correct level, and 4 for what I'm about to eat. Which usually works. But if it doesn't absorb properly (tunnelling? Is that the same as pooling?), I guess what I then eat will continue to raise my BG without the insulin to go with it.
I don't think it's scarring, I've had a pump for 18 years, but I rotate every 48 hours from left stomach, left leg, right leg, right stomach....and so on. So it never stays in the same place very long. And when it happens, like yesterday, I move the site immediately to rule out a poor infusions site/poor absorption. Also I use Novorapid in the pump anyway so its quick acting.
And regardless of what I'm eating/doing, the key point is that ALL the insulin I bolus EVENTUALLY does it's job - so it's not that the insulin doesn't go in at all, it's the length of time it takes to act which is very different to a normal reaction. So yesterday my Freestyle Libra graph results look like a mountain!! It just goes up, and up, and up in a very straight line, and then when it hit 24 yesterday, it decided it was finally going to take notice of all the insulin I had bolused over a 2-3 hour period - and then came down in an equally straight line (regardless of what I then ate over those 2-3 hours).
I think this could this be tunnelling then - and maybe I should not eat/drink anything at all til I see it start to come down?