Damp found above the hatch from outside to the locker under the bench seat. Obviously this ingress should be coming from above which means either the window (which I did last year - proper job ...) or the roof. However I also suspect that it may well be getting into the locker through the seal and sucking up through the wood that way as there is no eveidence of damp immediately below the window.
So fist I sealed the roof. This I learned about via a Facebook post several months ago. The technique is quite simple - remove the stuck out bit of rubber between the aluminium strip and the flat of the roof. Clean and then mask a narrow band between the aluminium and the roof. Fill with sealant - black Soudall fixall. push firmly into place with careful smoothing using a tool. Remove the masking to leave a clean edged black strip. It looks quite good. First attemp was on a length of roof about 6' long. I did this on quite a cold day and will do the rest. I just wish I had known about this when I had the whole of the front edge of the roof off a few years ago. It may have saved a lot of time and Im still not sure what I did then worked. I'll do the rest at some point soon to match and prevent further problems but removing the older bodge I did will be a struggle.
Second job was to go at the hatch which after several attempts I managed to fully remove. The damp and some rot were now obvious, but all I did was to put it all back again, this time properly sealed. When it was removed there was potential for water to get behind the extrusion and seep upwards in the wall. Extra screws and carefully applied sealant should do the job.
Update 1 February 12 2018. Very warm weather meant I could make progress with this job and I finished the rest of the roof side, all round the front and the passenger side too. All looks very neat. I haven't taken any pictures because I haven't cleaned the rest of the roof and it looks a mess!
Update 2 February 22 2018. Following a trip to Silverdale, where it rained solidly from Tuesday evening all the way through to the early hours of Wednesday morning (30 hours in total), with only a little respite for drizzle, the damp near the locker seemed less obvious. Also some traces of damp spotted near the driver's door repair from last year had disappeared, and there was no sign of ingress around the passenger window which I had never fully got sorted despite previous multiple attempts. I am cautiously declaring this a success.