Floor Insulation

Glastoun

We have suspended timber floors with an air-brick void underneath, no crawl space access. I'm pretty sure that the cold air underneath is removing heat from our lounge, and also that some of the cold air is making its way through the carpet - there are black marks around the edge of the carpet from where dirt has been filtered out of the air below, and if I lift a small corner of carpet there's a very noticeable draught of cold air coming from between one of the floorboards.

Is there a way of insulating/sealing without taking the floorboards up? I've read about sealing between the floorboards and skirting, then laying thin board over the whole floor and then underlay/carpet over that, but would that make a noticeable difference if there was still no insulation under the floorboards themselves?

Anyone else had similar experiences, and what worked for you?