http://www.old-maps.co.uk have quite a repository of old OS maps covering most of the UK. The neat thing about it as I understand is that you can move the grid over the map and select that area for download on payment of just £10 per tile. This is against £20 + for a print version. The OS charges considerably more I believe. It is worth considering if needed has anybody tried this service out??
In my rummage through their site (Old Maps) I have found that there are a couple of things to remember particularly when searching. Names and county names have perhaps changed since the period you maybe searching. Rather than search for the quarry or mine, search for nearest town/village and then crawl to the location.
The OS have now released their own beta Map API. Note the terms and conditions. This is a welcome step but we will no doubt have to wait and see what the full cost will be for commercial/semi commercial applications. It is based on the OpenLayers API so can be used along side other API's. To give you an example of what I mean you can see my map server here
http://www.map.toadstone.com note: use the blue/white + to the right side of the window to change layers.
I have an OS API licence number and will be putting up an OS version layer in due course. One guy has already done a fine example of what is possible (I'm just an amateur). You can see his site here:
http://wheresthepath.googlepages.com/wheresthepath.htm This is along the lines of memory map. You can zoom out to any part of the UK. By using data layers, added to and drawn from a database, you can create very useful tailor made maps.
Peter