I went to this a couple of weeks ago. It was a flying visit as I had my 3 month old with me but I really enjoyed it, highly recommended for those of you close to Winchester.
It is only 4 pounds entry and if you gift aid that then you can go back as many times as you want that year. I didn't get a chance to look around the whole museum but there is more there than the Peninsular to Waterloo exhibit. It looks like it follows the rifles from AWI to modern day.
The exhibit itself has one room dedicated to the Peninsular war, including a brown bess and rifle (that you can pick up and mess about with) as well as some uniforms (including a scrap of napoleon's coat...) various other smaller items and a lifesize model of a rifleman. The diorama has been very well done and includes a presentation that highlights parts of the board and discusses what was happening at that point in the battle.