Which tradesman to ask?

Jane, thanks but there is no visible mould so if that is the problem it's lurking unseen in the cavity maybe.

PS I did worry son was getting covid 19 as loss of smell sense is an early symptom. However he can smell spice cupboard, rubber hot water bottles, and even a plastic shoe horn and seems other wise in good health.

I will not obsess about the smell and possible water ingress as there are people who do not even have a roof to cover them, or cannot pay their rent or mortgage. My reason for keeping the discussion going is some other gran might have more experience of this sort of problem.

My son cannot smell anything bad and suggested I am imagining it. I think an experienced roofer will be the best bet to go up in the loft space to look for water ingress and dead animals.
Dead animal would be the good news as that would decompose and that would be the finish of the smell.

I recently had some mould on the bottom window frame in our bedroom. I just sprayed it with some all purpose cleaner, left it a while and then cleaned it off with a J Cloth, it hasn’t returned thankfully. I have made a point of opening the bedroom windows every day since. A mix of 50% bleach and 50% water in a spray bottle will work as well.

Thanks annepl.

Alexa it may not be that. I'm just giving you my experience. But fingers crossed it's nothing serious.

annepl, that is reassuring. I have lost sleep tonight imagining builders in for weeks with a large open gap in my main kitchen wall.

I have looked up the internet and it seems the smell is mildew which can be quite toxic. Also that it's caused by wet around the brickwork. I was imagining a disgusting puddle inside the cavity wall.

There are companies and tradesmen that deal just with window issues.
I have used them a couple of times to replace handles, and deal with a foggy window, and they were very reasonable.

Get some local recommendations.
I would go with the joiner or window specialist first and if they couldn't do it , a builder.
I ask my ladies at the knitting group. They know all the best people!
Good luck with the mould problem.

We had damp caused by the brickwork not being sealed at the window. Simple inexpensive job to fix it. I can't remember who fixed it but it definitely wasn't a double glazing firm..