Always a big problem, David. You do need major access to the cistern. To change or repair anything (trust me, it will happen), you need to get all around it.
The neatest one I did was when there was a bedroom on the other side of the wall, with a built-in wardrobe. I put a large access panel in the wardrobe. Simple job to get at the whole works from the rear, without disturbing the tiled front in the bathroom.
That doesn't happen very often in a lifetime, though. Another way is to site an access panel right in front of the cistern... before fitting the WC pan. You have to align the edges of the panel exactly with the tiling joints. Then temporarily hold the panel in place with duct tape.
Tile over the whole wall and panel. Drill through the panel tiles and screw (with cup washers on the screws) into pre-positioned timber battens.
Grout the whole thing. When you need access, dig out the grout around the panel and cut the tape with a sharp blade. Then lift out the panel.
Do the business, then re-screw the panel in place and replace the bit of disturbed grouting.
Either that........... or ditch the tiles altogether and use detachable timber ......... much easier :o)