Superimposed hearths over combustable materials (which is what you're
proposing) are allowed, but the Building Regulations specify is some
detail what is permitted.

Simply putting a hearth on to of the wooden boards would not be
permitted unless it was very thick, or had a substantial airgap
incorporated, or was a type of stove that had been approved as not
causing excessive heating underneath.


You might find it easier to remove some of the suspended floor and
extend the constructional hearth.

See Approved Document J:

http://www.planningportal.gov.uk/eng...000000503.html

(page 30 - I've needed to look at it myself in the last half hour).