I called the vet's emergency line, and arranged for a vet to come out and look at her and take x-rays. They came and gave her some drugs to fight the inflammation, and also took x-rays. They think the laminitis is due to mechanical reasons - the hard ground and that she is without shoes (her hoof size and quality isn't the greatest) - rather than the grass, since the soreness got slowly worse with time rather than coming on suddenly. She is getting a bit older - she's probably about 15, and is a bit heavy - so insulin resistance may play a part as well.
She's on stall rest for the moment, and twice a day anti-inflammation drugs by mouth (luckily, she's very good for this). Once we know what the x-rays show we'll have a better idea of her prognosis. The irony was that she was due to get her front shoes back on tomorrow - we'll likely hold off on that since she's pretty uncomfortable and the vet would like to get the inflammation under control before we shoe her. I feel bad that we didn't get shoes back on her earlier - poor girl! Please keep Maisie in your thoughts - I'll post tomorrow when we know what the x-rays show.