Try drilling a 3mm hole at the lowest point of the back silencer box. When not run often, condensation accumulates there and can take ages to evaporate. The hole can be plugged with a self tapping screw and a dab of exhaust sealant for the MOT. It also allows the water to escape if left out and cuts the internal rotting of the box. This may not be your problem but if you are not losing coolant, it could well be.
