[oldgrunge]

Further to my earlier post, were one of my three melon plants was looking sickly. That one has virtually had it, and it's two companions are following suit. Does anyone have an idea what the problem is, please?

[Plot 1 Problems]

I'm assuming your watering and feeding them regularly so it's not nitrogen deficiency.
It could well be  Fusarium Wilt, a fungal infection. Sadly there's no cure for it so it's the end for those plants if it is the cause.