Its just suddenly struck me that I have not considered the rainwater harvesting freezing in the winter. The overflow pipes are OK as they are never full, but what if the hoppers freeze? The snow melt would likely overflow above the ice in the hoppers and down onto the concrete base and under the cabin, potentially causing damp/rot issues.
The DPM the cabin is laid upon (in addition to the one in the concrete base) is lipped up at the back where the hoppers are located so any water collecting on the "shelf" would be diverted away from the pressure treated bearers, so that's not all bad, but a direct overflow out the top of the hoppers may cause damage to the external walls.
Also, the final downpipe of the gutter is below the water line, so the melt water may back up the downpipe and the gutters themselves may overflow. That ones easy to sort though.
Hmmm.... Anyone know anything about this stuff? I guess I could insulate the hoppers with foil backed rockwool and gaffa tape...
[edit] I have just had a look at it and I think if the downpipes into the gutters are above the water line and that water freezes, the melt water collected by the gutter system would flow over the ice in the hopper and into the overflow system anyway... so long and the melt water does not exceed the capacity of the overflow pipe it should be OK. I think... I will cut the final downpipe section off to a few centimetres above the full water line. [/edit]