Soil Pipe Leak or what?

I am afraid is serious dig it all up time mate.

Get a CCTV survey.

I have three pipes going from the house to a trap (gully, soil stack, downstairs loo), one piple from that to a second trap covering a 90 degree bend, and a pipe from that across the garden under the fence. All five pipes are cracked and some are displaced. All perfectly visible through the CCTV survey.

All of the damaged sections can be lined. I have no personal experience of this but I understand it involves the insertion of a sleeve impregnated with epoxy, into that is then put a cylindrical balloon which is inflated with hot water under pressure, which forces the sleeve against the pipe walls and holds it there until the epoxy goes off.

Possibly the socket of the clayware drain pipe receiving the base of the soil pipe has split. They do quite often, (that is if it is clayware and not PVC drain),

Gain good access through suspended floor and check. The socket is usually left standing proud of the sub-base.

Is the soilpipe Cast Iron, Asbestos or PVC?

if you can get the wife to flush and the see where the water is coming from then you could fix with 'plumbers mait' or similar as the soil stack is low pressure the joints dont have to be that good to get a seal, it maybe just a joint that has moved, if not then it will be a dig.

hth