Well, the block floor that's lifted is banjaxed anyway, so if you can ease it out and begin some investigations. Is the skirting board behind the rad sound, or has greyish mould taken over at its base?
I think the skirting will have to come off anyway, so if you're up to it begin the happy process of prying it away from the wall. This will give you a clearer view of the supporting wall and the floor underneath.
Is this floor wooden (suspended) or solid?
If solid, seeing below is going to involve digging. You could ease up part of a suspended floor for a look.
What about the other side - have you had a good close look around the washing machine and soilpipe to see if ought is amiss?
I'm beginning to think 'house insurance'......