The doors you are referring to are called pocket sliders. They are very expensive around 180 a pair, although as they're meant for joinery, the cabinet maker would need to fix a 16-18mm jamb and create a recess for them to slide back into, this would effect the opening access of your doorway. I don't think these are the best or easiest solution, however they could be adapted to your needs. Personally, I think the FOC Face of wall sliding door is the best solution, given you have at-least 900mm on either side of the door to accommodate the open door?..if you don't; then, without knowing your house style these may be way off, how about bar doors, the name escapes me the ones in western movies where they're either side and you push through them. I've see them in a solid core door, so at-least they close themselves, and you could teach your cat to nudge through them, so its tidy when they're shut.

A floor to ceiling cupboard would work well in this space. As mentioned in the above comment using Mirror on such a large surface in the bathroom could become a hassle to constantly keep clean. However if you do like to look of Mirror I do suggest a dark tinted mirror, as this would go quite nicely with the colour of your tiles and not look to harsh in the bathroom. For the doors a Pocket door system would work quite well! this is where the door is opened and then you can slide it back to be in line with the cabinetry. I would keep the colours of this cabinetry quite plain and stick to the same colour palette of the tiles. Goodluck !

disappointing that your landlord is not more responsible and more difficult if you're at work and your partner is not well: if there's no legal agreement about who owns what and you've been told to "deal with it" disposing of anything that you can't use or is not obviously valuable...it's not safe to raise the washing machine without a specialised washing machine stand and it will be more comfortable left on the floor to use the top of the machine as work space... try the simplest cheapest options and keep only the essentials on a shallow lower shelf and hopefully the top shelf may have room for some of the culled boxes from the bedroom...check out the various shelf options in the on line Bunnings etc catalogues ..unfortunately the paint can't go in the bin so make sure the lid is secure and with the help of some hefty friends carefully enclose the paint can in plastic garbage bags (in case of spills), load it in the boot of the car (supported by something to keep it upright) and take it to the nearest recycling garbage tip...hopefully the worst that will happen is the owner will need it replaced if/when she wants to do more painting.. if you need an oven it could be worth finding out the cost of a repair but it could also be no more costly to consider buying your own convection microwave and use the old cleaned out unworking oven for storage of some of the stuff out of those boxes...for the rotting kitchen bench there's a product called "builders bog" that can fill in all sorts of horrible problems and then (carefully covering all the other surfaces use a cheap can of (red?) spray paint to at least make the bench look a bit better and, one box at a time you can take control of the situation, best of luck to you

Thanks siriuskey for your design and comments. A bit of background would no doubt be useful, so apologies for this late addition to the discussion. The house will accommodate a reasonably fit and active retired couple in their 60s (us!). We have elderly Mums and expect that 1 of them will end up moving into the guest room/study (hence the desire to have a connection between this room and a shower/WC). We'd also like to be able to access the backyard and have visitors able to access the WC - although we accept that at times we can't have everything...although it's worth a shot! We've struggled with the layout of this space for some time now and picked our builder's brains, but so far no-one has been able to come up with a solution. Both oklouise and you prove that other options are out there.