If they're Calendula type marigolds, they may well have seeded themselves, but they won't grow till next spring. If they're the Tagetes (french) type marigold, bin 'em.

the larger African ones like the French ones are not hardy either.

Save a few flower heads. Place the flower heads in an envelope and store in a cool dry place. Plant the new seeds in spring. They will grow quickly and bloom all season. good luck.

I agree with all of the above but have you noticed how things survive all winters when growing at the coast & when you go there in January they are lovely big plants & in full bloom, especially ordinary marigolds. Don't know where you are in the UK, Patsam.