Help - hole in wall and damp! - feels like a nightmare

I'm refurbishing my bathroom...(installed by previous owner I call mr bodge-it... for very good reasons...)

The wall is cavity (from approx 1970) and seems to have been cavity wall insulated.

I'd noticed a damp patch on the wall and realised it was where the redundant toilet overflow was...from the outside I could see the pipe had been cut off level to the wall...on the inside a patch of lumpy filling/plaster approx 20cm round. I thought the pipe hadn't been sealed off and rain was getting in. I was going to seal the pipe with a bit of plastic bag and expanding foam...

I chipped the filler off to find that brickwork around the pipe is missing - it had been badly filled in with 'new' plaster. Behind this plaster around the pipe was bits of broken brick and motar for the rest of the width of the inner leaf and into the cavity and there is approx a third of a brick stuck in there.

The pipe itself was stuffed with bits of old wallpaper but it was dry inside and no sign that it had ever been wet/damp......the pipe was actually pretty loose (taken out and put back before?) and came out with some of the plaster...

So what is causing the damp? Could it be the brick bits or old pipe acting as a bridge? Is it rain water coming in around the outside of pipe? What should I do?

Should I put the pipe back?
I thought put expanding foam around the outside to act as a seal and then fill it expanding foam?
..or leave the pipe out and try to fill the hole in the outer leaf with foam from the inside?

And on the inside -can I fill the hole with foam? Then cut back and skim with plaster? (Or will this cause a bridge too?)

(It is very difficult for me to get to the area outside...I don't think I have a long enough ladder! and access inside is limited because it is close to the current toilet)

Thanks for any advice ...the whole refurbishment is turning into a bit of night mare

Ok - I know that was a long one...
Someone I spoke to has just suggested this to me....does this sound ok?
(also suggested damp might be caused by condensation on a cold spot)

Leave pipe out -
Fill outer leaf carefully with foam. Next time someone comes with a ladder -borrow it - or get them to fill the hole with mortar.

Get as much debris out the cavity as possible (inc the wedged in brick) and stuff with fibre glass insulation and then fill inner leaf in with foam, plaster etc...

Can anyone see a problem with that?

Thanks...

that sounds about right but make sure you use cavity batts not normal loft roll insulation as it is not water resistant.

@neo

Are you saying that batts are water-resistant as standard ?

PS Did you ever have anything to do with magnets ?

Well the industry standard rockwool batts are.

Not quite sure what your on about regarding magnets??

A very powerful family of magnets were developed in the 80/90's using neodymium which was usually abbreviated to neo, hence my question.

PS What do they actually say in the spec as to their ability to repel water ?

Ok I've sealed the outer leaf and cleared the debris out
(the 'third of a brick' was a nightmare - actually the back off one of the bricks in the inner leaf must have broken off as he chipped at the front...I had to make the hole bigger to get it out )
Have stuffed normal rock wool in there to stop draughts (or children posting things down there) for now until I get out to buy some cavity wall stuff ...
Looking at it it seems like I'm going to have to get a huge bit for just the little I need...currently using polystrene sheet insulation for something else...have lots of bits of off cuts etc - is there any way I could use that instead?

I wouldn't see any problem with using your poly off-cuts, but personally I would leave it unfilled for a while to:

1) Dry out
2) Ensure that there is no further leak