(1) Liszt: etude 'Eroica'. THere is a page of double octaves. How best to practise these? Should there be any movement at all of the wrist? And is my 5th to be always straight or can it 'bend' slightly?
I do not play this piece

, so I do not feel I can give specific advice about it. I will however say this:
1. The wrist is a hinge. Therefore its main role is simply to accommodate the movement of the hand and the forearm (which it links). When people talk about raising wrists or lowering wrists this can lead to all sorts of confusion (unless the movement is being demonstrated) because what you raise or lower is not the wrist but either the forearm or the hand. The wrist itself can only do two things: Be “braced” in a number of positions, or be relaxed and pliant. For example, if you shake your hand loosely (as if to shake water off it), your wrist is relaxed, the hand is allowed to shake freely. If you keep your forearm and your hand as one piece, the wrist is braced and firm. This “braced” and firm position can be such that the back of the hand and the forearm make a straight line. Or you can angle your hand up (what people call low wrist, but it is actually a high hand angle) and have the wrist braced (this will stretch the muscles on the underside of your forearm). Again you can angle your hand down (people would probably call this high wrist position, but it is nothing of the sort) and in this case if you keep your wrist braced, the whole hand-forearm will act as one piece and you will be stretching the muscles at the top of the forearm.
2. My view is that the wrist should only be braced at the moment of contact with the keys, and at all other stages it should be relaxed. Although you can brace the wrist in any position from the highest angle your hand can manage, to the lowest, I would advise against such extreme positions being used consistently: these are the positions that will lead to injury. Reserve them for the very rare occasions when only such positions will do. Bracing the wrist at the level position back of hand and forearm on a straight line) is the most efficient, safest and possibly most generally useful position. Again , such bracing is only necessary at the point of contact.
3. I suggest you experiment and find out what works best (Liszt himself was said to “shake octaves out of his sleeve” suggesting that either he played octaves with a completely relaxed wrist (which I find difficult to believe) or that he only braced the wrist for a fraction of a second when making contact with the keys (which I find far more likely). Hopefully someone else can give you more specific advice.
(2) Right/ Left hand. I practise mostly hands separate but invariably, my right hand tires very little and my left hand is the opposite with the result that the right hand gets more practise, which is the opposite of what i'm aiming at ANy suggestions most welcome! ?
I said that elsewhere - cannot remember where, so I will just repeat it:
1. The main reason (for right –handed people) that the right hand seems so much better than the left hand is because in normal life they use the whole right side more. Consider eating with a fork. It may look like you are doing it with your hand, but you are actually moving the hand towards your mouth with your arm. If you try to eat with the left hand, you may think the awkwardness has to do with the hand, but actually is the arm that is not used to the movement.
2. Likewise, when working on the left hand, we are unable to use the full co-ordination of the left side (since it is not so much used in normal life), and therefore at the piano we tend to stress the left hand fingers, forgetting that the whole right side is being used in playing, not just the fingers. This is the main reason why the left hand tires before the right hand. It is not because it isweaker, but because one is using it differently.
3. Consequence of the above: If you want to improve your left hand, practising twice as much with the left hand is not going to help much. What is going to help is to use the left hand for everyday tasks for a full month: eat with the left hand, use the mouse with the left hand, open doors with the left hand, write with your left hand, you get the idea.
4. Observe carefully what you do with the right side of your body (not just the right hand/fingers) and try to do the same on your left side. This will be difficult but following the suggestion above (doing everyday things with the left side) will be of amazing help. It is surprisingly easy to become ambidextrous, but it requires attention and consistency of use for a limited period of time.
(3) Height of piano. This is a rather peculiar one and probably concerns very few of you. Given my height (almost 2m), for most pianos my knees are stuck at the level of the keys and cannot go under the piano (as normally). Lower seating helps to a point but the most logical (and not the easiest) solution seems to be a higher piano (which makes a problems with the height ot the pedals!). Anyone encountered this also?
Try lifting the piano on wooden blocks (brick size). Figure out the exact size of the wooden blocks that will lift the piano ideally for your height. Have them cut at a carpentry shop ( you will need four), and carry them with you so that you can lift any piano to your size. You may have to adapt your foot to the heightened pedal though (or have another block for you foot).
I hope this helps,
Best wishes,
Bernhard.