Posted: 01/06/2017 at 15:28
Try again next year. Sow the seeds in March, indoors, then when they are about two to three inches tall and have second or even third sets of leaves, prick out into little pots, separately, not to big pots for a start, then keep in the GH or a plastic one and keep potting on when the roots have filled the pots.
keep them in for the winter, and by the following Spring you will have a big robust plant that should have made it into a 6" pot, tough enough to withstand slugs, plant them out at the start of May and they will flower for you.
if you want to grow perennials well, it's a waiting game, prepare for two years ahead.