That's fungus and you need to fix it quickly or it can infect other parts of the camera, or even the lenses you use upon it (if it hasn't already done so).
I would imaging in London you have some local camera repair people. Get the camera to them ASAP. They can tell you whether it makes more sense to replace the entire sensor assembly (after checking the rest of the camera to be sure there isn't fungus there, too). Or, there are folks who remove the AA filter for modification purposes, such as to convert the camera to IR or to leave off the AA fitler entirely for even sharper shots (but at the risk/cost of increased moiré in your images).
There are many types of fungus in the world. Some of it grows pretty quickly, likes dark and damp places. In camera gear it can permanently etch glass and ruin optical coatings. I don't imagine it's good for silicon wafers that camera sensors are made from, either.
If it were me, I would also be looking closely at any other gear you keep with the camera. Funguse can spread from one piece of gear to another. It sort of "blooms"and gives off tiny spores. I don't know if there are fungicides that can be used with camera gear to stop it. Usually it means disassembly and thorough cleaning of any infected bits with something that will kill the fungus and remove traces it's left behind.
To help prevent future problems you may want to get some sort of moisture absorbant product to keep with your gear. Not all fungus needs moisture to thrive, but a lot of it does. A packet of desiccant kept with your gear may reduce the risk of reoccurance. Hunting supply stores and others often sell a reusable type of desiccant that changes color when it's saturated with moisture, then you put it in an oven on low heat for a short time to dry it out and can reuse it over and over.