They look to me as though they've been scorched by the sun. they don't look very healthy at all. I live in Derbyshire as well and have clay soil , i sympathise with your dad .

could be the awful rain we've had and lack of drainage in the pots or ground-- and the slugs love that sort of weather
you can get copper rings to protect vulnerable plants, If you want to grow them in pots then bigger ones would probably be better-- with some drainage and maybe sharp grit on top to deter the slugs, I find that bigger pots are easier to look after as they dry out slower and can make a lovely show with the plants grouped together ( have a look at pts & tubs on our Goypedia pages

just read a tip in the Telegraph--- mix rock salt with vaseline and smear around the rim of your pots to prevent slug and snail damage-- hope it helps your Dad, get him reading Goy-- he may enjoy it here !

I think they are/were primulas perhaps?

Thank you so much for all your help and ideas. I'll pass everything onto my dad, and yes i will suggest that he becomes a member himself, he's often on the computer, i don't think its a step too far for him!