Damp at base of wall, DPC above internal floor level

[dormermike]

I think i may have made a breakthrough with this. Can anybody advise if the following sounds plausible please?

To set the scene: The front bays are not insulated, yet all the other walls are. The base of the bays is the worst for damp, apart from one wall in the front room either side of what appears to be an old fireplace, now bricked up. Beneath the fireplace, is a cavity, which extends under the solid floor, about a foot deep and a foot and half wide approx. Ash pit I guess.

Anyway i managed to scrape away some mortar to view into the cavity near a damp spot, and I can see the bottom of the polystyrene insulation board, and also the exterior wall. The exterior wall is dry, which seems odd considering the interior of the interior leaf is damp. The insulation board stops just above the DPC, which itself is above the floor level and above the damp.

The damp bricks are noticeably colder than the dry bricks, and the floor above the cavity (ash pit) is stone cold.

I'm wondering, is it plausible my damp problem is actually condensation caused by the lack of bay insulation, and the hole under the floor making the slab very cold, and the wall polystyrene board not extending down far enough?

I'm considering filling the cavity as the rest of the floor is solid. Would it be advisable, and would expanding foam be ok? From what I can see the inside of the cavity is bone dry, its just the top that is very cold (the room side of the slab on top of the cavity, aka the floor).

Not much i can do about the cold bricks at the bottom of the wall - i am loathed to get polystyrene beads pumped in. I can see the top of the polystyrene sheets as the top of the cavities are open, but i can't push them down any further.

Looking forward to your thoughts..!

Thanks
Mike