Having done some research (I assume you're talking about Cestrum nocturnum) it seems its not possible to keep this plant at a certain size by root pruning or severe pruning of the top growth. Pruning appears to be restricted to removal of dead stems, which should be taken back to the main stem when removed, and clipping off spent flowers, preferably before they form the very poisonous berries. Further, if the roots become restricted, the plant will die, so potting into larger containers yearly is essential.
An interesting fact about this plant - its fragrance appears to irritate the airways of asthma sufferers and there is some doubt over whether the chemicals producing its fragrance are toxic in some way.

Thanks for the reply, Yes it is a Cestrum Nocturnum and yes the fragrance was very strong at night, If I brought it indoors, You would have to open a window! Although usually during the day It was fine (Either way, You will notice the fragrance!) And I was aware that every part of the plant is toxic, (Thats why It was on a 6ft high wall out of reach from children) As with your answer, It seems to imply that I have to replace the plant each year, Because im told the plant won`t survive the winter here in the U.K, And as it would be too heavy to put it in a larger tub, That would seem to limit my options, I could see if it lasts indoors to the spring, Then plant it in the garden to get one more year I guess, Or then perhaps? wrap a fleece around the plant to make it survive though a Devon winter? Like my other Jasmines, Although apparently a Night blooming jasmine, Isn`t really a Jasmine at all, So perhaps not?

Well its worth a try, isn't it? They are tropical and won't survive our winters (specially not this one) but I mean you're in Devon, and apart from this and last year, we usually have much milder winters. I realised it probably wasn't possible to pot it on again, given its in such a large container already. I'm wondering about cuttings, not sure how to do those for this plant, but if you could start some new ones, those would take a while to get to such a large pot and be easier to move in and out of the house. Thinking about it, some sheets of glass around it against a wall with fleece might give it a better chance - I use old glass to protect tender things on my balcony, and it does help enormously.