DIYable to plasterboard porch?

FreeBear

stuart45 said: Moisture can soak through the brickwork which is why vertical DPC's are put into the jambs in situations like this. 

But the property wouldn't have had a porch door originally, so no vertical DPC. To retrofit one now would be time consuming and relatively expensive.

stuart45

Not originally, but when the doors were fitted one may have been installed.
If not, as you say it's a lot of work to do it, but if any damp does find it's way through in that area he will be aware of a possible cause.