Thank you all so much for all your wonderful suggestions, I shall have to try some of these out. Definitely look up some evergreens as well. I'm growing dahlias from tubers at the moment, so I'm looking forward to seeing them flower! Funnily enough we used to have a helebor that we dug out and we have replaced it with lupins that we're planning to collect seeds from. Thank you for all the suggestions!
... some of the ones mentioned - hellebores, some viburnum. Some cyclamen. My neighbour put some bedding cyclamen in in Sept they went straight through the winter in a mild spot. My abutilon was covered in flowers all summer and went past the frosts in a sheltered spot.
Depending on where you are I would echo the suggestion of Hardy Fuchsias. Once they get going they will flower until hit by frost, though with milder winters I have seen them flower straight through from one year to the next, but obviously you cannot guarentee that.
Winter heaths (Ericas) are quite long flowering for winter/spring, ours started in late December and is still going now, and looking like it will probably just about reach the end of April before it stops. Won't add to your summer/autumn display beyond foliage (which isn't bad either) but they make for a nice mound of pink on otherwise grey days.
Helenium 'Sahin's Early Flowerer' produces orange/yellow streaked flowers. If you deadhead it regularly you will find it flowers almost non-stop from July to October. It makes a wonderful (contrasting) partner to the dark blue of Salvia 'Amistad' mentioned by @Mrs_Grohl. It also looks good with red-flowered cultivars of Persicaria amplexicaulis (listed by @Cambridgerose12).
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Funnily enough we used to have a helebor that we dug out and we have replaced it with lupins that we're planning to collect seeds from. Thank you for all the suggestions!
Winter heaths (Ericas) are quite long flowering for winter/spring, ours started in late December and is still going now, and looking like it will probably just about reach the end of April before it stops. Won't add to your summer/autumn display beyond foliage (which isn't bad either) but they make for a nice mound of pink on otherwise grey days.