Damp and Mould Issues in Terraced Property

I became a member today and, although I have read many articles, this is my first post. Can I say, in opening, what a valuable resource this is. 

I have one BTL property near Barnsley which is an old (1885) terrace. I bought the property in November 2013 with tenants already in situ. The Home Buyers report said no damp was found.

On a visit just before Christmas I was told of a small damp patch in the corner on the floor of the upstairs bedroom. We agreed to monitor the situation and they would get in touch if it became a problem.

Having now read some articles I realise it was always going to ‘become a problem’!

However, I heard nothing from my tenants on this issue so when I visited again last Monday I was shocked to hear of their experience – there is black mould in the upstairs bedroom in the corner and under the window, mould under the downstairs window at the front (NW facing), peeling paint on an internal wall and the chimney breast had been damp to touch. The bathroom has also got mould – there is no extractor in there so I have asked an electrician for a quote to fit an Humidistat fan.

The tenants reported that they have been wiping the mould off and repainting or wallpapering over the affected areas. I know this is not the way to deal with it.

My question is what is the best way to tackle this issue?

Do I purchase a DIY eradication kit or does it need a proper investigation to find the cause?

If so, how do I find a reputable damp specialist working in the South Yorkshire area?

I am very keen to resolve this issue for my tenants as soon as possible.

Many thanks in anticipation.

Lynne