I went and had a good look at the tests, and whilst the 'positive' line is much lighter, the control line is also somewhat lighter. Not heaps, but some. Hubby commented he could see that too.
As I said, I used my two fresh packs (bought last week) first to ensure accuracy early on in this cycle, and the box I opened today I bought around April, when the weather was still hot.
I wonder. This will give me some hope for a few hours anyway. I think I'm going to have to hold for a few hours and get some fresh tests.