Hi, I used wooden panels on mine - but then being a cabinet maker I wood! (sorry!)
Obviously if you use wooden panels access is easy to arrange. If you use tiles the quickest way is to cut out a square in the backing material that exactly matches up with a number of whole tiles, fix a frame behind it, and then replace the bit you cut out. This'll leave you with a removable panel in the backing material.
When you tile over you should (if you've measured right!) be left with a removable panel with perfectly aligned tiles stuck to the front. Around the edges of the panel use silicone instead of grout.
When you need access cut thru the silicone and pull the panel out.
You'll need to replace the silicone when you replace the panel, but if you don't plan to remove the panel often that is fine.
Silicone will hold it an A4 size panel in place on its own, but if you want a larger panel, or want to remove it more often, make the frame a bit bigger and use screws through the tiles to secure the panel. Still use silicone round the edges, but you won't need to replace it when you refit the panel since the screws are doing the holding not the silicone
(Tip. You can drill the screw holes with a masonry drill, but a special tile drill is MUCH less likely to break the tile)
(Second tip: for smoothing silicone buy a â€œfugi, a really expensive plastic smoother available in plumbing merchants (not DIY stores). Beats the heck out of wet fingers, spoons, spatulas, or indeed anything else people who never have to do it recommend)
If you've gone to the expense of the wall hanging kits (ouch!!) then it's quite likely that your supplier does the furniture / cabinet to go around them as well. Might be worth a look.
If making it yourself there's nothing wrong with Oddbods suggestions at all except if you are going to tile you should use 12mm or 18mm WPB Ply not just any old bit of timber.
I find mirror screws are a good plan for securing your access hatch. Nice and tidy! Oh and DO USE a proper tile bit rather than other types of drill bits - the wastage if you don't is phenomenal. Never use hammer and run the drill on the lowest possible speed. Before you drill break the surface of the glaze carefully with a very fine sharp instrument (just a light tap will do) where you want the center of the hole before you drill and the bit won't skid around horribly. Masking tape doesn't work!
Many thanks for your replies. Sorry I haven't them acknowledged before.
Initially my builder tried to persuade me to buy ready made units but apart from the cost, we preferred the look of tiles. In fact, I only came across two suppliers who offered units to accommodate 'off the wall' WCs: Ideal Standard and Howden, neither of which were suitable.
On this occasion I let the builder arrange the 'boxing in' since I have more than enough decorating to do and my partner wants it all finished by xmas. 3/4" marine ply was used for the panelling and battening allows for removal for access in two places, much as was suggested. However, the plumber had never fitted an 'off the wall' WC before and I feel that I could probably have done that aspect of the work better myself.
Incidentally, I eventually located a British manufacturer of wall frames and they also supply the frameswork to which pannelling and tiling can be applied.