Here's pictures of my garden!

[FuzzyCustard]

A huge improvement on the overgrown nonsense when we arrived 2 years ago...but what would you do if it was yours?
Clues:
Heavy clay soil
Windy, coastal site
Celandines everywhere!

And what else can I grow in the mossy bank (under an oak tree) to extend the spring flowers season. (I've got bluebells in my head)

That looks fantastic Fuzzy. For your mossy bank, how about Lily of the Valley, violets and wood anemones?

Thanks JT05. I am transplanting violets by the dozen (like weeds they are, but very pretty - they're in every plant pot!) and love the suggestion of Lily of the valley and wood anemones. I'll be looking out for those.

I don't have young children but young grandchildren - only occasional visitors and none of them have any interest in the garden! (And cats)
I have some self seeded forget me nots elsewhere but think they would prefer it sunnier? Once the oak tree leaves come out it is pretty shady, so stuff has to have done its flowering by the end of April. (in a normal year!)

What about muscari for your mossy bank? I have some that have appeared and colonised a rather dark mossy area - on very damp heavy clay. Quite long-flowering too.

Loving the mossy bank! I’ve just discovered
www.plantsforshade.co.uk
(....and have a rather large delivery arriving tomorrow, only ordered Saturday)

Hi OP, I have clay and shade (though not costal). What about something like Vinca minor (Bowle's Mauve) which likes shade? It's also low maintenance (flowers May-September). If you want winter colour - anything from white/yellow to dark almost black - try hellebores. They look fabulous and do well in a shady woodland garden. If you're after bulk, ferns are also a winner - and they come up every year too.

Thank you all for your lovely ideas. I shall be acting on them shortly! It's really helpful to get other people's ideas and know what has done well for you all too.
Cheers m'dears!