If the unit was bought in the UK, and you told the sales person what headsets (and type of aircraft e.g. open-cockpit, microlight etc. ) you wanted to use the Intercom with, then you may be able to return it under the Sale of Goods act as being unfit for it's purpose. You might have to argue with them that the level of background noise makes the unit truely unfit, but most companies will consider your arguement.
This is definitely a purchase that should be made only with a guarantee that it can be returned for a full refund if found to be unsuitable.
Did you try the unit out before purchasing it?