Just updating this in case it is relevant to anyone in the future.
After the flood (which was clean rain water), we peeled back the carpet and underlay to try and "hang" all of the wet material, whilst also exposing the floor boards. We placed 2 dehumidifiers in the room (which we had already), running day and night. In hindsight, we should have bought some of those mopping up sheets (scroofix do them); it would have got the worst of the water up more quickly. Or even bought a wet'n'dry vacuum, I think they can be had for £40 something.
Anyway, we must have emptied 30+ Litres out of the dehumidifiers over a 10-day period. Slowly, the boards, carpet and underlay started to dry out fully. Initially there was a smeall of damp, but when it was all dry it didn't smell at all. We left the underlay in place, and could in fact of used the old carpet but the teenager had painted the room and got paint all over it so we got a new carpet.
Everything is fine now - no smell or any indication that anything was wet.
Having the heating on 18 hours a day, and running the dehumidifiers, probably was a longer and expensive way of doing it.