There's a massive difference between full sun and semi shade, and whether it will be in one or the other makes a big difference to what plants you choose. Are you wanting permanent planting?

Is it for someone who will look after it or do you need low maintenance?

The lighter heucheras will not like full sun, but the darker ones will be happy.

I shall be looking after it so want low maintenance please because the tub is at the bowls club I belong to but not within walking distance. I don't yet know which of the half dozen tubs I shall be allocated which is why I can't say which position. I'm hoping to find out today. It's only for the summer season though I think longer term planting would be nice as other recreations go on in the winter.

I'm not sure about perennials which are always my first choice but I could always pull them out and transplant at home if I decide not to go with geraniums and annuals. I'd like something a bit different to the usual tub planting.

My half barrel will be mostly in shade so I think I will go with heucheras around the edge. Haven't come up with a centrepiece though I'm leaning towards a dwarf shrub of some kind. Also I'm not sure whether to put in a few trailing plants.
All advice welcome.

The most low maintenance thing I can think of is Euonymus fortunii, preferable Emerald 'n Gold which is hardy, cheerful all year and needs hardly any maintenance. You could put something else round it (Heuchera eg Blackberry Wine?)until it gets big enough to fill the tub.

I was also thinking about a euonymus but silver queen as a contrast because I thought the gold might not stand out enough.
I also want a few trailing plants in between the heucheras but something with colour. Blue perhaps. Given the shade though I haven't come up with anything yet. It doesn't matter if they are annuals though they usually like sun.

My Emerald 'n gold is brilliantly coloured just now - a real ray of sunshine , but Silver Queen is great too. Blue lobelia would look nice with either?

Again I thought of blue lobelia, Stera, but given the shade it might not flower for long enough.
Today I bought Heuchera lime marmalade and Heuchera forever purple. They will be alternated around the edge. In the middle I've got Euonymus francien. I arranged all the plants before I bought them and francien looked best of all the available options for the centrepiece. I'm still struggling with trailing plants and now wonder if variegated ivy would look ok.
Thank you for all the suggestions. I'm really grateful.

The golden form of creeping jenny (lysimachia nummularia 'Aurea') is shade tolerant and will put up with dry soils between waterings. If it looks tatty, just sheer it back, water, and it grows back without any problems.

Lobelia, trailing or otherwise, grows much better in shadier situations than it does in full sun anyway, so use that.

I didn't know that, Bamboo. Thank you. In that case I might choose that. And thanks to you Andrew for your suggestion as that gives me another option.

Late last night I was trying to picture the barrel and realised that it didn't need all round viewing as it is against a fence so my arrangement of the plants will be different. Now I want a plant with strappy leaves to go at the back where I shall also put the euonymous. I was thinking of yellow Sisyrinchium which spreads rapidly, of course, but that doesn't matter. Any more thoughts anybody?

I'm beginning to get fed up with trying to sort this out so I shall be glad when I've finished it. I won't be satisfied, of course, but once planted at least I can't tweak once it's done.

Phormium 'Back in Black' - a dwarf variety growing to around two feet.

Yep, totally agree with Andrewr's suggestion...unless you prefer yellow, in which case, Phormium Yellow Wave, a little taller

Well I couldn't find a dwarf phormium so I took a chance on a dark red hebe though I'm not really satisfied. I know that likes sun but it will only be in situ until the autumn. I shall bring it home then and plant it in my garden. I've had enough of trawling garden centres now. And I never thought I'd say that!

For the trailing plants I chose a blue lobelia which I won't plant until late May.

I also spotted a dwarf buddleia, deep pink, which I've wanted for a while. That's now in a pot in my garden.

Thank you all so much for all the suggestions, they've been very helpful.