So for the last couple of months something has 'not felt right' when soldering up PCBs. I'm lead-free and use a solder with a 227degC melting point, and run my iron at 340degC.
Last week I decided to do something about it and splashed 19 GBP on one of these, which arrived this morning...
It turns out that, across the range, my iron was running between 40 and 50degC low.
A quick re-calibrate, by setting two points at 100 and 450, and at least the iron display now matches the thermometer display.