In reply to goosebump: You say you're getting them on the NHS, is this from a doctor, podiatrist or physio. Having spoken to a few people who have seen podiatrists it seems insoles are their first point of call.
I saw an NHS physio a few years ago who said I had flat feet and gave me good 3/4 length insoles made from a cast of my feet. This was up in Stirling but I've seen a lot of people who've had insoles from my local health authority which are pretty poor by comparrison. If they try to give you a foam insole with a bit of support glued into the arch then go somewhere else.
It's always worth getting a second opinion, having recently seen a really good physio (again on the NHS) I've been able to do without my insoles now. Apparently I don't have flat feet, it was just muscle weakness which had caused my arches to fall. If I'd known that earlier I might have had less trouble.
Ben