The deal says:
Save up to £100 on any Bose speaker dock when you trade-in any old audio product, excluding MP3 players and home cinema systems. Only available via Collect from Store orders.
BUT I can only see a saving of upto £50 - 4x products to choose from with various discounts using this offer.
Obviously we are talking about a more expensive range of speaker docks, but if you want to pay extra for better quality then I think this would be a good deal! Unless of course you can find cheaper elsewhere.
Hope its of use to someone anyway.