Damp near back door

An internal wall adjacent (at a right angle) to my back door (UPVC) in the kitchen has damp. I've had a couple of guys out who have claimed its rising damp buy sticking their meters in the wall etc. and trying to charge me for a DPC. I'm no expert obviously but disagree with what they're saying. The UPVC door was fitted just over a year ago just before we bought the house (1930's build). It is fitted on what looks like the original concrete/stone door sill/step which protrudes out externally. Internally I have pulled back the lino under the door and have now seen that the sill is damp and crumbling. Water does seem to settle on the external sill and drains slowly if at all. Is the door/sill likely to be the cause rather than rising damp. The plaster has started crumbling off now furry white salts are on it. Hope that all makes sense.