Old Ordnance Survey parallel maps

A few months ago, I found a link on this forum to a wonderful web site which displays scrollable old and new OS maps side by side. Sadly I forgot to save the link to my favourites and now I can’t find it! I’m sure some of you use it all the time, I would appreciate help finding it again.
Many thanks

Perhaps you meant this: http://www.ponies.me.uk/maps/osmap.html

Wilpin: this wasn’t the one, but many thanks because it will be useful anyway.
The one I found had two panes, side by side, the old map in one pane, the new in the other. You can scroll either one, and the other moves with it. It was especially useful for tracking old rivers and streams that have been redirected or built over.

You might mean http://wtp2.appspot.com/

Thanks Wilpin, that’s the one.