Ivy is the obvious one Catlin or you could go for something a bit more decorative that flowers very early in the year, like a Clematis armandii.

Do you think you could edit your profile to show us what part of the UK you are in? It does help to know if somebody is in, to use an extreme, John o'Groats or the Scillies.

Pyracantha wins my vote as a great plant to hide ugly fences. If you trim it frequently you can make sure it is self supporting. Or you could use wire on the fence to tie branches to as they grow up. It is evergreen, has masses of pretty flowers in spring and red, orange or yellow berries in the autumn to feed the birds. It is a bit thorny and would deter anyone from coming over the fence. Smaller birds will nest in it and if not they will certainly roost in it.

There are some excellent evergreen cotoneasters that have lovely white flowers in spring and red berries in winter. They're selk supporting after they had some support for a year or two ( ie when the stems harden up)

http://www.flickr.com/photos/31559373@N00/3554081853/

Also there are some very hardy evergreen honeysuckles

http://www.flickr.com/photos/31559373@N00/4760266453/

Yes Cotoneasters are great or how about climbing Hydrangea. Lovely winter stem colour, even more colour in spring with lovely green foliage, followed by flowers and then another show as the leaves turn in the autumn. NB they will grow well on a north facing wall. Roses like Queen Elizabeth grow about 6' - 8' high and if you prune them carefully they will reward you with a pillar of roses from top to bottom. You could put in espalier fruit trees.

My climbing hydrangea is not evergreen and the stem will not hide an ugly fence Scotsgran and it is perhaps a bit slow growing. Also, roses? espalier fruit trees? evergreen? Not round here.

If you are north facing on that side Camellias are good fillers. Evergreen....do not get out of control....if they get too big, just cut back. Bloom from December to May, depending on variety. You may have to give them a special feed if any leaves turn yellow. They respond fast to that

Climbing hydrangea is not evergreen and it won't blot out the fence but it will make it more acceptable. Ever seen a lady without her warpaint. That's the difference. You have to add interest. It depends on how you treat it whether it grows fast or not. No matter what you plant Catlin you need to dig out a hole at least twice the size of the root ball you are putting in it. Add some bonemeal to the bottom of the hole and fork it in. Sprinkle some bonemeal around the edge of the hole. Place your plant in the hole and back fill to leave it with the top of the root ball on the soil level as it was in the pot. Give it a thorough watering and keep it watered throughout the first year to give it a good chance of surviving. You do not say how big the space is and how much you can afford to give over to the camouflage. If you have the room Camellias would be my chosen shrub for early flowering rather than rhodendrons. Privet hedge gets a bad name but if you plant as a single shrub it too is evergreen, has flowers later in the year, which perfume the air all over the garden and look fine as a single bush as part of a shrubbery.