"Rachel Aitch" wrote via GB, not direct...
Quick question: one of my clients (I'm a professional gardener, but
don't hold that against me) asked me why his entire potato crop, of
various types, turned to mush as soon as they were put into boiling
water.

I have no idea!!

Desperate to preserve my reputation, I turn to you (picture me with
hands out, pleading) for suggestion...

Grown in too dry conditions.
And/or Wrong variety. You don't say which variety but floury ones will do as
you say especially if they have been subject to drought conditions. Try some
waxy ones and/or drought tolerant ones next year.

Kestral (SE), Spey (MC) and Romano (MC) seem Ok for us.
--
Regards
Bob H
17mls W. of London.UK