Thanks for all the replies, apologies for not getting back sooner.

The lawn is appearing in clumps as suggested, quite evenly distributed. Some parts (out of sight on that photo) are okay, but not great. The grass that remains is very course to the touch.

The house is brand new, on a new development, and the 4 houses yet to be occupied also have hideously long lawns now, too.

The excessive rain we've been having has clearly fuelled the growth spurt, as the house builder said they'd been maintaining the lawns. It was around 6 weeks where it was left to do it's own thing and something hasn't gone well.

I will investigate if there is rubble or debris under the soil where the lawn once was. The ground is fairly wet still, but drying out.

The front lawn is showing similar issues. The tops of the blades of grass are green, but lower down the blade it's yellow and damp - I've trimmed back quite aggressively, and raked the lawn to remove anything dead. I've probably done harm, but couldn't see it being any worse.

I'll also look into some seed and sand to improve it. I've never felt such a rough lawn before - is there a risk of me getting seed that develops different variants of colour/patches?