What to sow following forgetmenots

[GingerKitCat]

I have load of forgetmenots carpeting my sunny flowerbeds as I like the combination with tulips in the spring. Last year I waited for them to die back, pulled them up and shook the seeds about liberally. I was left with huge gaps and I'll be in the same position this year

Last year I half heartedly planted annuals but I wanted something slightly taller this year, the rest of my garden is quite 'country garden'. I have loads of seed packets - would cosmos and calendula work? I'm rubbish with poppies (see other thread) however I could chuck some into the mix
I've tried flower magic mixes in the past and found them patchy and disappointing - have they improved?

The forgetmenots are still in full bloom so it's looking like June before I get a chance to sow.

In summary I'm looking for something reliable and quick growing, any suggestions?

I planted nasturtiums a month ago but they're for specific pots/baskets as they grow rampant. I also have some sunflowers on the go indoors (bit late on those)

Foxgloves, self seed, but are usually bi annuals. You could get some plants as starters. As you say Cosmos would fill the gap, they overtake the space in my garden, so I avoid them. Like you, I fail at poppies, so the Cosmos might work well.
I have a little white flowered crainsbill geranium, which self seeds like mad. I don’t know it’s exact names as it was already here. In other gardens I’ve seen Love In A Mist, Nigella, become a self seeder. It might work.

Aquilegia looks great and self seeds. Whirling butterflies (Gaura) flowers all summer long and looks fab and comes back, verbena is tall and returns each year. I'm no gardener, but last year I bunged those in and this year I added to them, it's looking fab.

Thank you both! I have tons of foxgloves, they're great aren't they

Gaura and verbena look good
I have a couple of aquilegias bought as plants, maybe I should add to them!

My erysimum Bowles Mauve are doing splendidly this year (I forgot to mention them) so something of that kind of flowering height 12-18" would work.

Aquilegia are lovely (grow like weeds here, my dad digs them out of his allotment puts them in pots and sells them on the front for 50p) but aren't right with the timing as they are already going over.

Nigella / Love in the Mist has self seeded in my garden this year, no idea where it came from, but is just coming out now as my Forget-me-nots are going over - see picture. Maybe no good for this year but a consideration for next?

The garden centres will be full of perennials which would get great for playing out now to fill spaces in a month.
I love Campanulla/Canterbury Bells and have quite a bit of that coming through at the moment.
Geraniums are good at filling a space.

Verbena Bon-something are fantastic for later in the season, tall delicate long lasting, and mine self seed everywhere and come back on the same plants.

I can't sow anything straight in the ground because the slugs gobble up the little shoots. So I grow cosmos in plugs, pot on, then plant out, and so far the pesky slugs have, touch wood, avoided them. So yes, I'd go for them. Good luck.