The Sunderland factory was at Whitecross Bay, on the northern shore of Windermere, which is where this relic is supposed to be. The factory built 35 Sunderland IIIs and overhauled or repaired many more. Anyone with access to Sunderland serial number records ought to be able to determine whether any were shown as sunk or scuttled on Windermere. I believe the factory closed in May 1944 and was later dismantled, though the site remains as a caravan park and sailing centre. If May 44 closure is right, post-war scuttling seems unlikely as — emergencies apart — there'd have been no reason for a Sunderland to have been there. I know nothing of sonar images and cast no slur on the diving group, but there's a 'tool' on Adobe Photoshop that would enable you to manipulate a photo of a Sunderland, or of a model of one, to appear not dissimilar to this.