Welcome to the site.
Best to get an on site survey/quotation done, this is a free service to you and you will get 3 quotes. Simply post your job onto rated people's site.
You are not obliged to go with any of the quotes- but ever job that's posted is paid for by the relevant tradesmen.if they contact you that means they are interested in your job. If one of them are awarded the job then great news for you both.
There are so many reasons as to why the damp can occur so I suggest you place the job formally on this site to get a damp specialist to visit your home and carry out a full survey. Receiving advice on this part of the site will never resolve your issue.
best of luck & I hope you get it sorted.
You need to look at the different ways it could be getting there . Is it low level or high level , if its at a high level look at the external wall for any cracks or mortar missing. If its at low level look at a breakdown of the dpc .
There are several options that you can take before calling in the specialist, there are several DIY silicone kits you can buy and follow the basic instructions it simple and easy to do and could save you thousand s of pounds.
Bespoke contractors LTD
Please answer these questions , is there black spot mould present ? Is there a plug point in the exact area? Is there a bathroom or any chance of plumbing to that area or on the opposite side of the wall ? Is the floor solid or suspended , both sides? Have there been any obvious alterations to that area, like moving a door etc.?