If you post my latest request for pics it might get worse - the thing is, its useless to give easy, soft advice.
The basic thing in all remedial damp work is to find the cause, and then fix it.
The good news is the flashings are re-useable and the roof & basic structure should be OK.
The floor tiles might be fine depending on the pics & does the slab have a DPM (membrane) under the concrete?
The pics raise Q's ref the main house floors & walls but come back to that when your ready.
Here'a a couple more pics showing the door wall lower down. Have asked my neighbour for a pic of his side, didn't just want to wave my phone over the fence as it looks into their kitchen on the other side of that fence haha.
I'm not opposed to having to go back to masonry to sort it out. I assume I can leave the other walls untouched? This is a utility room and the other side has a sink, the boiler, washing machine and a few cabinets fitted now. It'd be a right pain to have to shift all that... It's probably going to have to be done in stages anyway because the outlay to get all this done incorrectly means I won't be able to pay for a fix in one go. Might even have to DIY a few bits if possible. At least it's summer, which is usually less damp...
Also not sure about the DPM under the concrete, they did pour some new though because they chased the old slab to put some pipework in for the sink etc...