At its greatest the kingdom extended at least from just south of the Humber , to the River Mersey and to the Forth ( roughly , Sheffield to Runcorn to Edinburgh ) - and there is some evidence that it may have been much greater ( see map ) .
At its greatest the kingdom extended at least from just south of the Humber river , to the River Mersey and to the Forth ( roughly , Sheffield to Runcorn to Edinburgh ) .