Pre decimilisation Simon, things used to go up by half a penny at a time. That option was removed when we went decimal as the new half penny was 1/200th of a pound, the old one was 1/480th. So immediately any time anything went up it went up by over twice the increases that it used to do and of course everything was rounded up as soon as we went decimal, so I reckon it cost us around 10% more in the first year.
Then the new half penny didnt last long so that meant the minimum that prices could go up was doubled again to 1/100th of a pound, almost 5 x what things could have been increased by in pre decimalisation days.
When we do change over to the euro, no matter what rate it is to the pound, I think it will again immediately cost us another 10% in the first year, the same as going decimal cost us