Alan, the clay soil and the run off of the shed will mean that the shrubs you plant will have a lot to deal with. Cornus Alba Siberica (or similar) will only give greenery and white flowers which will cover the shed in the summer but the coloured stems will look good in the winter. Cornus will thrive in the heavy, wet conditions.

I agree with Jimmy but you could also look out for Physocarpus although it is deciduous. It comes with lime green or dark red leaves. It’s also quite tough.
You could also consider Mexican orange blossom, Aztec. It grows to a tallish bush, flowers from late April and has a strong perfume. It’s evergreen in my garden.

Thank you both. I'll be visiting a garden centre soon, so will keep an eye open for those suggestions.