A perfect spot for a lavender hedge - but choose a smaller variety such as Munstead. Will need clipping back twice a year, usually around June/July and again August/September, after flowering is almost done both times. Take an inch of foliage at the same time to help prevent woodiness - lavender has a tendency to become leggy and woody with age, and will do that sooner if not clipped in this way.

I agree about Munstead. I have a hedge of it in a similar position doing really well ( smothered in bumble bees )and now quite a few of my neighbours want one too.

Trailing nasturtiums like poor soil like this too and are very colourful and last well into the autumn.

many thanks bamboo and inverglen for your replies. How tall does Munstead grow? It sounds like that would be a really good solution, is lavender long lived do you know, as it is expensive to buy.

Gets about 1.5 to 2 feet max - you could also use Hidcote instead, have a look at pics to see which ones you prefer, some are more vividly purple or more grey leaved than others, but always check the height and spread.
Cheapest way to do this is to order in spring from somewhere like Spring Reach Nursery (online but you can phone them too) - they usually have small plants, around £3 each. Depending on the spread, for 18 feet you'd need around 9 to 12 plants, but ask the nursery, they're very helpful. Lavender isn't a slow grower, so by the end of the summer, you'd have a reasonable hedge. Alternatively, check online for hedging plants - suppliers of hedges sometimes have lavender and they're cheaper because you're ordering a job lot, so to speak. Buy small plants though, they settle in better and last longer - lavender is usually replaced after 5-8 years because its got leggy, but you can raise your own replacements from cuttings - they root easily.