Linzer Delikatess potatoes not flowering

I'm growing potatoes this year for the first time, and since we don't have much space I've devoted one 2m x 1.5m bed to a few earlies only and decided to try Linzer Delikatess. I had a couple that I couldn't fit in that I put in a corner of another bed. Of these 2, one came up a good week or so after all the others (I'd given up on it), and now ironically is the only one with any flowers. The rest, while all growing away seemingly healthily, show no sign of flowering. We planted these in mid-March, and they are supposed to be ready in 80-90 days, which is about now, so I would expect to see flowers by now.

Now, one or two of the plants are starting to yellow. They don't seem to be diseased, so it looks like they're starting to die back without having flowered. Under one of them I can actually see a few potatoes pushing up out of the ground, which I lifted and then earthed up the roots a bit more (might be due to a mole which has been trying to get under the spuds and pushed them up!).

So I'm not sure - should I just wait until they die back, whether they've flowered or not? If they don't flower, will they even die back? Can I just start harvesting now? Does anyone know much about this variety?