If the springs are Britpart, this is your problem, they are very well known for sagging, badly, after about 6-18 months. Of course if you have loaded up the truck more heavily then that is another reason, but you don't mention it so I assume not.
If they recommend you to fit +40mm that's fine, it will lift it, but then standard HD springs should lift it to, from the knackered springs that are on there.... Personally I would fit either genuine, which aren't that exp[ensive actually, or some Bearmach, which seem to have a better reputation.