If only you had a bit of your front unpaved. Clematis galore are at your disposal. Perhaps some small evergreen bushes also, box, ivies, or dwarf conifers.

There are hardy evergreen Jasmines and Honeysuckles that climb and will also flower. NW sounds a good aspect for them. As well as the above, Christmas box can work well and gives off a nice perfume in winter.

But all will do better if planted in the ground - put them in pots 2 feet deep, and you're immediately virtually at the top of the railings... see my other answer under your roof garden question.

Thanks for the suggestions - I now understand most climbers need deep pots. Even shallower root clematis like Clematis x Cartmanii (Joe & Avalance) which was suggested on another site?
Think I will stick to combination of box, shrubs and lavender. but any ideas of how to inject more colour?

I wouldn't have said that Clematis x Cartmanii was particularly shallow rooted. If you leave some space in your pots or containers you can add annuals. Also small spring flowering bulbs will add a lot of colour.

I've just measured the area and a 2 ft deep container would come up to about half way up the railings.
If I got a large trough 2 ft deep by the length of the fence (2metres or so), anything evergreen suitable to grow up and over the railings?

Appreciate all your comments

For a north west facing position, Lavender is not a great option. Buy Senecio (now called Brachyglottis) 'Sunshine' instead, it's got grey leaves. You can have colour just from using different coloured foliage plants - as you're planting in pots (which will, of course, need regular watering) use ericaceous compost and plant Pieris (loads of varieties available, some smaller and variegated leaf forms too), Skimmia (again, lots of new, smaller and colourful varieties). Euonymus 'Emerald n Gold' is a colourful shrub year round, doesn't need acid soil, ordinary potting compost will do, or Euonymus aureopictus, which grows more upright. If you're not in a cold part of the country, Photinia Little Red Robin is a dwarf form of P. Red Robin which will do well in pots. All plants mentioned are evergreen.