Ok, looks like I do have to buy a sensor cleaning kit. Firstly, I took a couple of shots of the sky and didn't see the spot. Just now I've done the proper test. Put it to the smallest f/stop on my 16mm and 56mm lenses which is f16. Set it to manual and infinity and pointed at a white wall, took the shots and moved the camera in a circular motion during the shots. In Lightroom, I went to visualise spots. With my 16mm lens attached and looking through the EVF, I noticed a circular blob, pretty much a perfect circle, situated in the top third and dead centre of my camera's rule of thirds grid. This shows up in the first screenshot. I then wiped the rear element and it's gone as can be seen in the second screenshot. The rest of the dust spots are the same for both lenses. They look more like water stains on the sensor than dust, don't you think? Would you say any of them line up with the spot on my tower bridge photos? You can click to enlarge the thumbs.
16mm with circle
16mm circle gone
56mm