Mouldy cellar - what tradesman do I need to fix it?

Everything in my cellar becomes mouldy. I've used a damp meter and one of the walls is off the chart in terms of dampness. I can't see any visible dampness, although admittedly, I have no idea what I'm doing. There is some ventilation coming from small gaps in the stone entrance stairs going into the house, above a small alcove in the cellar - however, water also comes in through here when it rains. And there's a "Tilt & Turn" window which I leave open slightly for air flow. Info about house: Built in 1890 Mid terrace - 2 floors Generally good condition I've owned the house for 1.5 years and this has always been an issue. I can't see any (obvious) evidence of damp at floor or ceiling level, just everything I put down there becomes mouldy. Who do I need to hire to assess and/or fix this? What would need doing? Thank you in advance for any help - I'm at a loss.