Cornishsally you've already asked this question and Bamboo has answered you - please delete this duplicate.

Sadly they really need to be sown very fresh. Indeed, you should collect them when they are still green and only just ready to separate from the mother plant. they have a life span of less than a month, so dried seeds are effectively dead. Next year, remove them from the plat as soon as they will do so with a light bit of pressure then sow in standard compost and put somewhere safe and they usually germinate the following Spring.
Actually if they like your garden then you are more likely to want to weed them out than to get more, they do spread if suited.

Thank you Owdboggy - I'll know for next year. I have still got a seed head with a couple of greenish pods on it, so I might try with that - nothing to lose! I don't mind if they do spread a bit as they are such a fabulous blue

Hang on Aconites are YELLOW? And the plans themselves would be long dormant.
Aconitum is blue (or white or pinkish). The seeds from them may be sown either now to over winter for Spring germination or saved dry and sown in Spring.

I just looked them up in the RHS book! It list aconites & monkshood under Aconitum. I didn't know there were so many colours. Mine are about 4' tall, I think they are Bressingham Blue but the label has disappeared.

Winter Aconites are Eranthis hyemalis. This is the trouble with using 'common' names for plants rather than the correct Botanical ones. Hard though when you only know the common name as most people do. Sorry about the mix up. To me, as I said , Aconites are Winter aconites and Aconitum are Monkshood.
Anyway, the advice remains the same. Dry store the seed in paper envelopes, not plastic, and sow in March. If your plant is Bressingham Blue then the seeds will not come exactly true to it, but still be nice plants.

Thank you Owdboggy. I'll try them in March & see what happens. They do surprisingly well in my garden & I love them next to Helenium in the late summer, so it would be great to have some more. I will be interested to see what colour I get.