I have only been learning the violin for about six months and have a plastic bow and a violin which cost about £50. Is there any point in getting a better bow at this stage as no-one seems to sell plastic bows so I wondered if that was because they weren't any good. Any advice would be welcome.

Any violin costing 50 quid, to be frank, is going to be bad. Personally I don't see the point in getting a new bow yet.. unless it's truly dire, wait until you need a new violin.

BTW, that doesn't mean rush out and buy a new one.. I've been playing on a £30 violin (with the same kind of bow as you have I'd imagine) for a year and a half now and recently got a distinction in my grade 1 exam - it's the playing that counts.. although, admittedly, I did have to get a new bridge and strings

By plastic do you mean fibregalss or carbon fibre? fibre glass bows can be alright for the beginner, its the hair on them that matters. if its nylon you won't get the same quality of sound as you would with horsehair, the rosin doesn't stick to it the same and so the bow doesn't sound the string the same. many people find wood preferable to plastic, but as a beginner it doesn't really matter at this stage. plastic bows are kind of a newish thing so i wouldn't worry about not being able to find one.
i would get a new bow when you get a new violin, and you want to get one that is well balanced, sprung and feels nice to play. its only when you start getting really good that you want to look for something really nice. and its like learning to drive a car, if you can drive a piece of crap a nice new car is so much easier, so its your technique that is important not the bow.
as ragattop said, its the music judges and examiners listen to, the intonation, not the tone of the violin, you only start worrying about that when you are doing concerts and performances.
so don't worry about getting a new bow yet