I was fishing this piece of really rough ground close to the shore for a couple of weeks but couldn't make any sense of what was on the sounder. I've attached a couple of images, the first being a 2D screen shot showing what seems to be an extra hollow lump on the rocky sea bed. The second image is my best side scan which again seems to show this solid return floating mid water. No matter what angle I went over it the images were similar.
Eventually curiosity got the better of me and short of going for a swim (water is only 6 degrees at the moment) I bought a GoPro camera and sent it down. To say I got a surprise was a huge understatement:-
https://www.youtube.com/watch?v=s64p259Poc0Turns out the fishfinder was spot on all along! My mystery object is the hollow, thinning boiler sitting upright, the highest part of a wreck that is in a position a quarter of a mile away from where it is reported to be in all the dive books. I wouldn't mind except I've been fishing the GPS co-ordinates of that "wreck" for years without catching anything. I'll trust the fishfinder above a set of GPS co-ordinates from now on.