@kenny Duncan: right now (and that's changing all the time) I have roughly the same amount of wine crammed in our space as I count in Sorrel's picture; but our under-stairs space is not half the size, only 7ft at its highest and L-shaped because the staircase is a winder. Without X-Ray vision no good picture, you see only the most-densely packed part of it. Not for sale :-)

Depending on what you're planning to do and type of subfloor , you should be able to hide all water pipes and some waste pipes if there is enough space to get the right slope etc. I think youn have a pipework for the radiator over there too ,which again you should be able to hide.

You could build out the whole wall to hide the difference. That's what we did over this doorway as it was a huge mess of pipes. Bold correction! . Good luck  Emily

I too have a boxed in cistern and we have had it tiled. We’ve had a similar setup for about the last 15 years. As Sarah L says, the access is via the flush plate. The first time we had this set up, our plumber actually made a couple of the tiles easy to remove by using a silicone around them instead of grout (in the same colour as the grout). It was never needed - and since we’ve had to redo the bathroom for other reasons, we haven’t bothered to do this again. I researched cisterns before doing this - I do recommend you use a reputable and reliable brand - I always choose Geberit, but I’m sure there are others too. Also cisterns come in different widths and depths - I went with Geberit initially because they have a cistern that will fit into a wall of depth 10cm.