Really excellent Dan.
Years ago Terry Wise did a series on Operation Sealion for Airfix magazine. I'm sure someone out there will have them. One part dealt with seafront defences, Guns hidden in fake Ice-cream kiosks and the like.
Can I recommend;
'The Last Ditch: Britain's Secret Resistance and the Nazi Invasion Plan' Lampe, D., Greenhill Books, 2007.
'The Battlefields That Nearly Were' Foot, W., Tempus, 2006.
For fortifications, the second book is pretty comphrehensive and well illustrated. The first deals with the 'auxiliaries'.
Dad's Army was, and still is, my favourite comedy of all time.
Don't forget the floral gun cover that Godfrey's sister Dolly made for the Smith Gun!