Spent yesterday fitting shelves to a cupboard that had contained an old HW cylinder. Walls were crumbling lath and plaster with nothing much to fix to, pipes running everywhere at different angles (the adjacent bathroom), and who knows what else lurking beneath the surface.
The solution I came up with was to make a series of boxes with 18mm ply tops, 15mm mdf sides and 6mm mdf bases and backs, the ply taking the load, and the 6mm base just keeping things square. I could then do the pipe cut-outs individually, and just stack the boxes on top of each other.
Worked out really well, and demountable if anyone decides to do the plumbing and electrics properly at some point. Probably expensive in terms of materials, and having the bits cut to size, but a small price to pay for predictability. I'm getting too old for those Hamlet cigar moments. "I'm sure I can hear a drip" etc