OK - it's done. Used the POR15 products and pleased with the results. Looks like all the salt and crap gets sprayed into this area by the back wheel and then gets trapped between the frame and the seat so a quick rinse with a hose doesn't get rid of it. No sign of any corrosion anywhere else on the frame. Surprised no one else has had the same problem. Oh well, summer's on the way?!?!?!

hi, had the same prob with my 3tb,wire brushed all the crap off,hammerite
spray painted the back half of the frame then cut a piece of linolium type floor covering to fit under the wheel arch,held in place with the seat bolts and cable ties..job done..