When I moved into this house in 1990, I found in the loft two
interlocking concrete tiles, and thought what a good idea for the
builders to leave a couple of spares.

I've never needed them, but today I'm going to shift the position of the
soil vent-pipe and reduce it from 110mm to 82mm pipe, to create some
extra wriggle room when re-doing the bathroom. I have a new zinc/rubber
'weathering slate' and hopefully I can do this without damaging any
tiles, but thought I'd get the spares out just in case ...

.... only to find they're not the same type of tile as on my roof anyway,
just some junk left behind by the previous owner!