Its actually better to leave the ground near fruit trees bare- in a narrow border like that any plants would be too near the trees really.

If you must have one, I'd recommend "Blue Star Juniper" Attractive sparkling blue foliage on dense, spreading branches. Creates a colorful contrast to purple and green foliage plants. Use in a rock garden or as a low border plant. Ideal for mass planting in minimal-care landscapes. Evergreen.

An old gardener told me to plant my rhubarb under the apple trees as they seem to benefit each other.

Steragram's right, there should be a clear area of at least a foot around the base of each fruit tree, especially for the first two or three years. Use a mulch instead to keep weed growth down and moisture in, but don't let it sit against the base of the trunks. You haven't said which fruit trees, but I wouldn't recommend any kind of Juniper if you're growing pears because of the risk of Pear Rust, and that particular Juniper develops a spread up to a metre, which will be far too wide for your border.

Maybe Vinca (Periwinkle) would be OK as it has quite shallow roots?

Yea, smaller stuff like Campanula portenschlagiana, Arabis, Aubretia and the like will be okay, but not for the first couple of years after planting (although I loathe Vinca, Sheilabub, messy grower...) but ground cover will make it more difficult to apply fertiliser and mulch as time goes by - being as the trees will be ruthlessly pruned and kept to shape, organic mulches and/or fertiliser are pretty much essential.

If its a new planting you might sprinkle a few small annuals such as Virginia stock or the one known as "eggs and bacon" whose Sunday name I forget. They aren't ground cover but are transient and shallow rooted if you feel you must have something and would be gone by the time you wanted to mulch. Are these stepover espaliers or tall ones?

I agree that no ground cover is better for the fruit tree as a ground cover plant in the same bed will compete for nutrients. Just add a few inches of rich organic compost but not up against the tree trunk. Give a few inches of clearing around the trunk. You can always place an attractive planter with summer annuals for a splash of color. Regarding compost/fertilizer, organic is a far better choice than the synthetic fertilizers like "Growmore" "Miracle Grow," etc. especially when it comes to fruit trees.

Thank you all for your suggestions......it sounds as if they will be better left alone with an empty border and just fed and mulched for the first years .
They are upright espaliers.
Thank you again