Thanks for the replies, we have now been and done it and are home.
Was it worth it? It was OK for free but I wouldn't pay extra for it.
Food was fine but service was poor and it's just an excuse to sell their wines to you, if you can get hold of a waiter to serve you. The setting of the Horizon buffet does nothing to make it feel special, it would have been better in one of the small restaurants.
We had a good time overall but we don't like Iona and are very disappointed with P&O overall, they have gone very down market and are on a par with Marella/Thomsons now except Thomson/Marella's entertainment is far superior.