insulation for underfloor heating

[grania]

I plan to install hot water underfloor heating to a 1900's mid terrace. Ground floor is currently wooden flooring laid on stone which in turn is supported by metal joists.

Insulation of the ground floor is key and I would rather over insulate.
I want to re-lay the wooden flooring.

What insulation material would you recommend and how thick?
Thanks for any advice as new to this area.