How worried should I be that this crack has appeared and it appears to flex a couple of mm when I stand on the stair next to it? Probably slightly wider than a £1 coin at the wide end. Stair 2 and 3 are now really squeaky, none of the rest above then are.
On a more serious note, it's pretty normal. You can try using a flexible filler like Toupret Fill-Flex. I've done that with a very fine scrim on staircases, really time consuming process but it's worked well enough.
How worried should I be that this crack has appeared and it appears to flex a couple of mm when I stand on the stair next to it? Probably slightly wider than a £1 coin at the wide end. Stair 2 and 3 are now really squeaky, none of the rest above then are.