I managed to poke that slot through the floor with my fingers.
I figured if I was lucky the rot was just due to water going over the edge and should be an easy fix... but the reason I put off looking at this too closely was I dreaded the worst case scenario, which would be floor under bath totally shot need to rip out bath and half the wall tiles with it, replace floor, get and fit new bath, taps, shower, tiles.
Good Lord! There must be spiders under here... and why are those boards at that angle?
Looks like my worst case scenario was right on the button.
I punched most of the floor away with my fist... Not showing off, it just crumbled away.
More cobwebs... I really wish I hadn't started this.
A temporary fix...
OK so next week I need to get a new bath. The problem has most likely be caused by the handles on the sides of the bath leaking. Does anyone know where I can get a bath from stock in the Twickenham Kingston...ish area? Just been checking on-line and the big DIY stores say 2 weeks to 28 days! I want one by next Thursday. The plan would be to get old one out Saturday new one in Sunday... and if I ain't done it by Monday I have help promised for Monday. So I need bath, taps, fittings by Thursday.