In reply to Beardyman:
I'm sure you already know the answer to this. The weather is very unsettled at the moment and any forecasting past a couple of days is likely to be unreliable.
Meteofrance will only forecast until Thursday, any site that claims to be more accurate is likely deluding itself.
If you're going to come, come. You just have to take the risk.
The climate is pretty much the same as the south of England, and I imagine you'd be hard pressed to forecast for next weekend on Dartmoor.