How much to fix dry rot?

I've got a spot of dry rot in my bathroom.

It appears to have formed in the corner at the end of the joist. There is also a joist that runs through the wall into my daughters bedroom. From the video it appears that the rot has just nipped the end of that joist also.

Meter readings state all joists are dry, showing level 11 which is well below the 20 level for dry rot to flourish. All bricks are dry and there is no leak. Dehumidifier reads at 45.

It appears that the previous owner had dry rot in the bathroom and some of that may not have been fully removed.

I can't fix any joists replacement myself, but any rendering or plastering is something I could do.

I live in South wales so I'm hoping it will be a little bit cheaper to get someone in. Anyone have a rough idea how likely it will cost to fix?