Having tackled the worst of the weathered sides of the wheelhouse, and on the hottest day of the year so far, i decided to take down all the clutter on the roof, and try to fix the leaks coming through it.
I took down the radar, the support strut of which was bolted through the roof causing it to leak
removed the old and rusted navigation light boards (i THINK this type of navigation light isn't even legal any more)
as well as an old pivoting searchlight mount, the ariel for the GPS thingy and a million feet of wire from the radar.
We were then left with the roof itself, the three removable panels of which (for if you need to collapse the wheelhouse to go under a low bridge) were covered with what turned out to be zinc sheet, not lead as i had assumed.
This would have been brilliant waterproofing protection, had various holes not been drilled through the sheets to mount the radar, searchlight and various other bits and bobs.
As it was we decided the best thing was to remove the zinc altogether, paint the wood with cuprinol for protection against damp, and then fibreglass the roof in one impermeable sheet. We took off the (very heavy, unwieldy) zinc sheeting with some difficulty, and painted the wood, and are now ready to put some nice flat plywood sheeting on top of the boards before finally fibreglassing the roof.