Damp bricks and soil under hearth

That is odd! It might be water that has come down the chimney and simply sat there if the fire has not been used for a long time. I'd be tempted to dig down, (Carefully in case there are pipes under there that you are not aware of!) and then replace it with concrete. Depending on depth it might be easiest to use something like fast setting post fix type concrete. That goes off pretty quickly saving time waiting for normal concrete to set. If it is deep and would be too expensive to fill with concrete, then fill most of it with dry aggregate. Sand or ballast or sub base will all be fine and save you a lot of mixing. If you find yourself in a situation of finding a deep void beyond what you dig out, then make a simple timber box to hold the filling in. I would also advise wearing water tight gloves. You never know what might be under there!

Hello, Whilst your survey may have shown no damp issues, the survey only relates to the day the surveyor visited. His or her findings are based on their observations and evidence they find. There could be more going on that isn’t easily identifiable, particularly if the weather has changed since their visit. It’s probably best to investigate this further, with a view to determining the root cause of the moisture you are finding. Simply laying concrete into an area which is suffering with damp may just cover up an issue, shielding it from view. This might make you think this the problem is solved, when in fact, the problem is still likely to be there, but could grow into something more serious if left unchecked. There has been a lot of rain, so it could be coming down from the chimney, but either way, you are likely to want to investigate this on a logical basis to get to the cause of the matter

I would defenfly try to use some damp course membrane before laying back..

Check chimney pointing and flaunching. Check you have a rain and bird guard installed. Storm dry your masonry chimney stack exterior so it can still breathe but it won't let any damp in. Complete a water test spilling water from top to bottom with a person at the hearth to see if it's still getting in. If after all this no sign of water you should dig down but be careful for pipes then lay constructional hearth as in building regs doc j and replace bricks then lay super composed hearth. Good luck with it all. More then likely be water coming down chimney from deteriorated pointing brickwork. But make to check for any leaking pipes or some thing outside nearby leaking as well just in case. Good luck.

Also it wouldn’t hurt putting some damp course in

Hi I would dig it out. Rub all mud of bricks. Leave for a couple of days.Then have a look at the brick for damp. You can always paint a damp proof sealer on bricks. Then I would back fill with concrete