Does this look like dry rot spores to you?

[SerenaOverjoyed]

DH swears this is dust from when we recently sanded our red-stained wood floor. This is in the cupboard under the stairs which did get some sanding..

There's a musty smell so I'm worried!

no, it looks like sawdust to me.

if it smells musty, take up some floorboards and check for damp. There should be a good airflow from the underfloor airbricks on one side of the house to those on the other. They should not be blocked, covered up, or concreted over.

Have you got a water meter?

Thanks for the quick reply. I agree it looks like sawdust, but you've saved my sanity.

We will take boards up at the weekend. I've taken the moth ridden carpet in there up and binned it.. can a moth infestation cause a musty smell?

We do have a water meter. What am I looking for?

when all the taps are turned off, look at the middle of the glass dial. There may be an air bubble. Is it still turning?

Dry rot looks different to this. It shows as horizontal cracks in the wood and then ‘beautiful’ lacy threads over wood, plaster and brickwork.
We once ( knowingly) bought a house with it all over the front elevation!

Our water meter doesn't have a bubble, but it does have a spinning cog which didn't stop when I looked at it this morning. People were in and out of the shower though - would I need to wait a while after water use has stopped to be sure?

yes