At the bottom of my garden is quite a busy road with my garden bordered by a fence and hawthorn hedge

The best thing I can think of is Honeysuckle. They are pretty tough.

Honey suckle and a clematis Montana, after a few months they'll have grown nice and thick, be covered with flowers and look ace!!

Though between them they may just strangle the hawthorn! What is the soil like at the base of the hedge Borodin?

Anything that you can realistically grow into the hedge won't act as a sound barrier. Only large evergreen shrubs, densely planted, will help with sound reduction. Do an online search to find suitable shrubs for your particular aspect, soil, and weather conditions.

I once saw a prog. On telly about trees as sound barriers. They need to be substantial woodland to make any real difference. I'm ever so sorry, but I don't think you'll be satisfied with the results whatever you plant. I do hate to be negative, but wouldn't want you to waste your time. Still, if you get a good thick barrier you might 'feel' better about the road...be less conscious of it if you know what I mean.

Fully agree with Cottagekarer. Since sound takes the path of least resistance, if you can see the thing causing the noise through your fence it will not be an adequate barrier to its noise. You will have to have some kind of absolutely solid barrier of an adequate height which might be impossible to have taking zoning regulations into account.

If you still want to plant something go ahead. At least you'll have something nice to look at.

Foliage and small twigs will filter out high frequency hisses and squeaks, but the only things that absorb or reflect that low frequency rumble are many layers of thick trunks, or solid panels of well anchored planks.
In summer, a fountain closer to the house would help distract from the noise.

I would say that the best situation would be to bite the bullet and replant the hedge, (if its not too long), with a robust evergreen hedge. Prunus laurocerasus, cherry laurel would do the trick but could become a beast in the garden. Why not try Portuguese Laurel (Prunus lusitanica), which would thicken up and be more attractive. You could plant in a zig-zag pattern to make it wider, thus making it more sound proof.

Thanks to all for your kind comments and suggestions. I realise it will not offer up much of a solution regarding sound (which is why we recently had secondary glazing installed - brilliant !). Primarily I wanted something that would stop the base of our hedge looking so bare during the winter months and give it some colour. Thanks again...

The bare lower parts of your hedge are probably due to lower branches being shaded out.
To alleviate this I suggest pruning your hedge with a sloping shape rather than with a vertical shape, meaning the hedge should be about half again as wide at the bottom than that at the top of the hedge so the front and back of the hedge slopes towards the mid-line of the hedge.
This will allow light to reach the lower stems/trunks of the hedge therefore allowing new growth in these areas.