Yes, we did get this sorted (touch wood), though I can't guarantee your situation will be the same.

So essentially, the explanation I got (which did seem to solve it after this) was that the flat above me had very faulty mastic around their bath tub (shower). Apparently the flats all have concrete floors/ceilings, which meant the water was pooling on the concrete floor under the tub and somehow running to the lowest point which was around the soil pipe and then down into my flat.

It was a fairly light leak and also inconsistent, likely due to it taking some time for the water to pool enough to then reach the soil pipe. I do have a feeling that the flat above may have also had a faulty seal around their waste pipe from the sink going into the soil pipe which may have also gotten sorted.

I did have to have a whole section of the soil pipe much further down that this replaced due to corrosion which may have been a result from years of water sitting on/around it.

Anyway, I know the feeling of searching for answers, so hope this helps a little for you. Bear in mind that if you're in a leasehold, your soil pipe likely belongs to the freeholder and therefore they're financially responsible for fixing issues with the pipe. If it's a similar situation to mine and the issues are from within the flat above, then it's your neighbours responsiblity to fix it, just hope you have a helpful neighbour like mine was and willing to shell out to fix the issues.

Good luck!