This sounds like chemical contamination; if some bleach got into the dev it would have that effect. I have never used Jessops but if course they do NOT have their 'own' lab as processing is a small part of their business; they sub-contract it out. I now use dlab7.com in Guernsey, cheap and very good. I have sent some very outdated Fuji 400 slide film which came back with a low d-max and cyan colour ( I was given it so did not know how it was stored ) but all other films have been perfect, including MY STORED outdated films; good d-max and colour balance.
-------------------- Got COMPUTERISED at last and now Digitised but FILM still RULES!