Ok. I think I figured it out.
The handbrake has gone (its the ratchet. I have the part but not the weather on my days off to fix it) As such I have been leaving it in gear when parked.
Leaving it in reverse when parked downhill seems to be the cause of the squeak.
That's a very strange diagnosis. One I've been pondering for a while.
Reading through your thread, you have said the noise is there when stationary, so the noise is not coming from the brakes. Therefore, using the engine as a brake is causing the squeak. I think it is more likely to be highlighting the problem, rather than causing it.
From your description, you appear to be parking it in reverse, when facing forwards downhill. This seems to be the recommended way, but I think it is dangerous to your wallet.
When parking in gear, we are trying to use engine compression to prevent the car rolling away. I've not yet found anyone to convince me that trying to turn the engine backwards will increase the effect of the compression, but turning the cambelt or chain the wrong way is courting disaster.
In normal operation, the cambelt (or chain) has a tight side, and a slack side. The tensioner is placed on the slack side, the tight side always being tight to maintain timing. Rotating it backwards makes the tensioner side the tight side and the untensioned side is now slack, ready and able to jump teeth. I suppose when valves and pistons meet the vehicle will definitely stop rolling down the hill. Although the owner's tears may cause a flood.
If this is what you are doing, reverse gear, facing forwards downhill, the you are changing the cambelt tension. Once running, the tensioner has to reset itself. It is unlikely that it moving on its pivot will cause much of a squeak. I therefore still believe that the squeak is likely to be the tensioner, the noise being warning of imminent failure.
Priorities:
1. Fix the handbrake. Getting very wet in the rain is cheaper than a new engine. It is also an offence for the handbrake to not be working.
2. Properly diagnose the squeak, and fix it before it breaks.
