yu are right - i was getting confused with the 1:50 000 ones.
btw can you still get those green fronted ones (path finder ?) which were 1:25000 and only about £4 but show a smaler area than explorer.
Finally I'd just mention that a quick google search ( other search engines are available
) for "OS explorer maps" brings up the 5.99 outlet in question about 5 or so down from the top of the page.