I have just had my internal cob walls lime plastered (hydraulic) and I am about to lay my oak floor once the plaster has dried out and the wood has a few weeks to acclimatise. The rubble walled cottage Im doing up has only just had its windows put in after 6 months without any at all and left open to the elements. Should I use a dehumidifer before laying the floor? Many thanks for any advice Andrew

top advice i'll get one kind regards andy

That's potentially a real recipe for wooden floor disaster - dry boards laid in a damp place that's gradually drying out is about the worst situation possible. You would lay the boards nice and tight; they would expand in the moist atmosphere, then shrink back as the room dries, leaving great big gaps between them.
I'd get the place as dry as possible before bringing the boards in at all. Dehumidifier, heating on/windows open, patience...
A prongie moisture meter would be useful if you could get a comparative reading elsewhere - do you know anyone locally with a similar building that's been lived in for a while, same type of plaster? Measure their walls with the meter then use that as a target. You can buy a cheap moisture meter of that type for £20 or so, which would have the advantage of allowing regular readings over time to monitor the changes in moisture level.
If it was me I'd think about laying a temporary chipboard floor and holding off the oak for a little while. I've buggered up a perfectly nice floor before now by being too hasty to get it down, and every time I've been in that house since (my sister-in-law ended up living there) I've cringed at the sight of it