Probably best to be a bit over cautious with young babies who are very damp intolerant, and will need to be kept as warm and dry as possible. IF the hutch is outside, keep it in a sheltered position and out of the rain if possible, I would be tempted to keep them covered at night anyway, as it has been getting pretty cold in some areas at night, and their little bodies are not good at adapting to temperature changes - keep them well insulated with hay.
During the day - if the rain is 'set in' - we lived in NW and generally once rain started it was an all day event ! - try and keep as much of the cage covered as poss - or bring them in if you have an indoor hutch - they really can't cope with damp or draughts/drafts - tried both and both look wrong! As 70 says, make sure they are ventilated as you don't want them over heating either - not much chance of that at the moment!
Hi, we have a lightweight waterproof cover. We bought it our hutch from pets at home and they make covers specifically for each hutch. When it was colder we put a duvet under the waterproof for extra insulation.
I think it is good to keep out the damp and will make the hutch last longer too.