1920s House Damp Crumbling Foundation - Cause?

Mcnoo

Hi All - I'm looking for some advice please.  I was doing some repairs to pointing of what appears to be a concrete slab along the front of my house.  But at one point I noticed that the render on the front of the house was loose and a chunk crumbled out and then I could feel that inside was damp.  I looked under the floor of the house and have discovered that the whole front of house is damp inside the foundations and all the mortar is crumbling.

Having read up about this it seems that a cause could be due to concrete cement.  I thought the culprit could be the concrete slab that runs along the front of the house but wanted to ask if anyone had any experience of this happening and what was the cause and also can I do anything myself (I don't mind hard graft) or do I need to bring in a professional?

I've attached some pics - one of the crumbling under ground, one showing the hole in the front house and one showing the concrete slab along the front.

Thank you in advance.