Rope wrote:

Removing the old tiles from the external walls, whilst
being a not to easy nor pleasant job, is doable. Removing
the tiles from the 2 internal walls is likely, I think, to
result in major structural mayhem.

Personally I hate tiled-over tiles, but that may be just me... it can
be, and is often, done though.

The 'structural mayhem' you describe is unlikely to be more than a need
to replace a couple of sheets of plasterboard, which isn't really a big
deal, especially if they are completely tiled over.

Is is preferable to remove the old suite and re-tile then
replace, or install suite and tile afterwards?

1. Take out the old suite
2. Remove the old tiles if you are going to - jut omit this step if you want
3. Fit new tiles

David