Has anyone heard of problems with filling underfloor voids with polybeads or even offcuts from eps EWI boards? Ive read a few of the other threads regarding this and most seem to say that it is a worthwhile exercise. Could it create problems with damp and rot as the ventilation will be removed?
It would solve the common issue with small crawlspaces under suspended floors.

Agree. Water vapour will (should be allowed to) migrate freely down thro the floor structure, which will remain at pretty much room temperature. Way below that, under a thickness of permeable full-fill insulation, will be the cold subsoil (and even colder surrounding foundation/walls). That's where the water vapour may condense, safely far from the warm floor structure, acting as a kind of dehumidifier for the whole volume.

By contrast, with void space instead of full-fill insulation, the void may well be full of saturated air, which may condense on the floor structure. Attempting to dissipate that air, standard recommendatuion is for copious through-ventilation, but in Spring and Autumn may well be importing air with even higher vapour content.

Before thinking of this, essential to ensure that there's no liquid water penetration into the void, via walls or up through the subsoil. And that the floor structure is sound, not rotten. One belt and braces way is to surround the building with a french drain down to bottom of foundation level, which intercepts any horizontal groundwater flow incl from pooling rainwater, and lowers the water table locally. So there's no liquid water, above subsoil level, to penetrate the void. Also cures rising damp.

Mice would love to live in that..!

The inside of the exterior walls is going to attract the condensation during winter, as they will be much less permeable than the insulation, and are colder than the ground, and are no longer ventilated. Could this be a problem for the ends of the floor joists if these are built into the wall?

Would you need to break out any concrete oversite layer on top of the soil, as this would attract condensation but restrict drainage?

Ideal would be to lay 150mm EPS boards vertically against the foundation wall outside face, within the french drain trench, from bottom of found up to just below external GL. That wd keep the found walls and joist ends warm. Of course if you're doing EWI, those EPS boards wd be continuous with, part of, the EWI. Super tea-cosy!

This assumnes you've got founds 750 deep or more - if it's an old building with shallow founds, then instead of vertical EPS boards, place that insulation as a horizontal bed of Leca 300mm thick x 900 wide, topped with topsoil over geotextile, a horizontal 'skirt' of insulation also serving as the french drain 'sharp clean aggregate' soak-down medium.

Thanks for the replies, this is very helpful. I'll let you know how I get on. I need to find a willing guinea pig now...