I have a Victorian townhouse, it appears to have been built without a
DPC at all, there isn't even a slate one I can find.

The problem I have is that the previous owners have had a chemical
damp proof course installed over much of the house and it hasn't
entirely worked. Some areas that have no DPC have no damp problem at
all... Most of the house is fine, but there are two areas (both
internal), one of plain wall and one around a hearth where there is
damp and chemical DPC.

I suspect the problem is very minor but is being made a lot worse by
the fact that a membrane has been fitted between the brick and the
plaster up to a height of 1m. This means that there is a line of damp
at 1m accross my wall.

Now, I am not sure if all the damp around the hearth is rising damp, I
suspect it might simply be rain coming down the chimney (not capped,
it fell off a year or so ago).

For the rest which is rising damp, I can see two, maybe three courses
of action.
1) Put in an extra layer of chemical DPC.
2) As I suspect the damp isn't serious, remove the membrane backed
plaster and simply paster over it with regular plaster, hoping that
the damp will simply evaporate.
3) Go along brick by brick, and somehow install a membrane DPC.

I'm replacing the fireplace around one area, so am tempted by option 2
there. For the wall it has to be 1 or 3.

Is option 3 viable? Is it ever done? I know it would be very time
consuming but I have a lot of time!

Now, next issue is the hearth itself, I'm fairly sure this is rising
damp and not from the chimney. Oddly enough, only the room side wall
of the hearth seems to have a problem.

Here I can see three options.
1) Put some extra air bricks in the external walls, although it is
already fairly well ventilated.
2) Take off the concrete and a couple of courses of bricks (then clear
out any rubble), insert some sort of DPC, rebuild (with DPC also
between the hearth and the internal wall).
3) Leave it all alone and hope that the presence of a fire will dry it
out.

Thanks for your help!