@ Yves, hank you for the photos, now I have an idea where to look for the fresh water drain.
@ David, good idea about cleaning out the muck but I think your heat exchanger his different from Gustavson / Lehman, will in any case investigate as last year’s new Stbd side anode in the header tank (Bosomotor) has disappeared completely.
@ Richard, the original ? Seems to have stirred things up a little ! all good stuff and very necessary. My response to your ? does not seem to have got through but I concur with David ref refilling. I would just add that leaving the low down drain taps open during the early stages of refilling until a bubble free stream of fluid comes out of them will reduce the possibility of an air lock in any lower galleries.