Thanks ladies for your replies. It settled a bit today but still in the upper reaches of heavy. I was lucky to be able to get in to see my lovely GP who has ordered some blood tests (thyroid apparently is implicated) and also prescribed the medication that BW mentioned. She said it's usually very effective so I'm hoping for some rapid improvement.

I also stocked up on maternity pads - the all-nighters I was using are actually the same as the maternity ones - but a different fabric on the cover - and stood in the shop and read all the tampon packets until I found the one that had the maximum absorbency in grams (carefree flexia super) so I feel appropriately armed now.