Solid floor - worth installing a floating, insulated floor?

[SiH]

Afternoon all
We're currently in the midst of what has accidentally become a renovation on a mid 19th century cottage. (9" solid brick walls)
In our lounge we have a solid floor of unknown age, with a more than slightly uneven surface, that the builders have 'Black-Jack'ed over as a precautionary measure.
They suggested that if we wished, we could lay a floating floor of sorts on top of this - something like a levelling scree of sand, followed by 50mm of Celotex type insulation followed by 18mm T&G chipboard.
The price we were given was a bit out of our budget, so we gave it a miss, and I am considering DIY'ing it.
So I guess a few questions:
1: Is the above a viable solution?
2: Is it feasible for a relatively inexperienced DIY'er to do (I have mostly done destructive, as opposed to constructive work, but I'm not opposed to hard graft.)
3: What materials would you suggest? The chipboard doesn't seem to be an issue, but finding the right PU/poly foam type stuff is a bit harder.
4: Above all, is it worth doing?!

If it helps, the room is roughly 3.5x5m. It will be carpeted using a decent underlay.

Floor height isn't an issue, as we have a roughly 4" drop into the room at the moment.

Thanks in advance
Si

 

[mointainwalker]

What was the old floor made of and were there any signs of damp ?

 

[SiH]

It's colid concrete, of unknown age, no visible DPM.
The carpet that was previously down had signs of damp at the edges, yep, however we are not sure of the source of that. The walls have had all of the plaster removed and are now clad in 90mm thick insulated board in an effort to eliminate condensation, and the house has had an injection DPC.