£300 is really not going to get you much in the way of monitors - the Genelecs that TO recommended are very good but sadly out of your price range. You may be able to find the budget end of the KRK monitors within your price range. They're active and come with decent enough reviews.
Don't forget though that monitors on their own aren't upto much cop - really you'll need decent stands (or iso pads) and some basic acoustic treatment for the room you're using them in. Which needn't cost the earth, but is worth budgeting for.
*Rahnooo*