That looks similar to my situation although mine is no where near as long as that. I agree that removing the tiles is the best way to go but i really like my tiles and I’m sure they would break if i started to remove them. As i said above i can access the void by removing the internal sill but until i do that i don’t know which insulation would work/be the best.
Was thinking of buying polystyrene beads and mixing with PVA glue in a large bucket and pouring it in, should take more than about 30/40 bucketfuls but just wasn’t sure how successful this would be. It certainly seems to be the cheapest option but eliminating a cold bedroom wall is the aim not saving money, can’t believe i just said that 😂.
The other option would be expanding foam, looking online there are semi-professional guns and canisters that are available for DIYers. This maybe a better option although much more expensive.
The only person who got all his work done by Friday was Robinson Crusoe.
Anything i post over three lines long please assume it is an article lol.