Recommendations to seal a leaking window frame

[ErrolFlynn]

I’m just looking for a quick fix, but I can see that attacking the outside first is best.

The thing that’s at the back of my mind is that the timber you can see in the photo will probably be coming off and tossed. It’s a ledger that helps support a deck (just out of view in the photo), the slate will probably come off too as part of the job, the main window you see will be getting replaced at some point, and the neighbour (above) has a water leak too which he thinks may be coming from the roof via the wall cavity. So, if the roof is the problem then my sealing possible gaps on the outside is not going to solve anything. Hence my original feeling of concentrating on the inside only. But, hell, the water could be coming in from multiple sources.

[rambunctious]

After rereading everything (apologies for not taking the time to read everything before) I would suggest you get access to the deck above and look carefully where the deck meets the wall because that is where most leaks come from,
so you might be able to silicone (with the owners permission) that join which may or may not help.

As to the window in the above unit leaking, if it is exposed to the elements, have a good look all around the window frame.
Even fly screens/security screens can cause leaks when water builds up behind them.
Being they are units, make sure when the deck is being replaced it is flashed/waterproofed to the wall otherwise it may impact you forever and a day.

This is probably the best quick fix you can do and basically all I can suggest without seeing what is above your unit..

If however, the upper window is the one in the 2nd pic then someone needs to get on planks/ladder and have a good look around that window.

[ErrolFlynn]

It's all good info/advice.

I particularly took note of the possibility of the above deck being a source of the water leak. That deck is to be removed, at some point. It's unsafe. There are rust marks from the bolt holes into the wall. If those bolts are a loose fit then water may be getting past them. A possible source.

There's a lot of work to be done, and those windows do take a hammering in a storm. So, any work done has to consider the weather.