Advice needed on 403 fuel tank cleaning

[Calibrator]

I guess a last resort is to do what I did with my 405. Nothing seemed to remove the hardened deposits for me, so I took it to a local firm who, for many years, have specialised in radiator refurbishment. A sideline is fuel tanks. They cut a section out of the top, cleaned the inside out, coated it, re-welded the aperture and painted the exterior. Could never tell it had been opened. £110.00 seemed very reasonable to me.