HNC/D are not vocational quals, they are not hands on. If your looking to upskill practically these are not the courses, level 3 is the highest you can go in plumbing. There is a level 4 NVQ but its a bit more general and looks at managing BS etc.
I would really recommend doing a NHC/D, they are tuff but intersting, it widens your scope, you think differently, can communicate more effectively with managers when they enter sites as you get to know buzzwords and where they are coming from, a new different way of looking at things. Im half way through a management diploma at the moment and have learnt so much, i didnt think i would!
Maths is the big stumbling block, i started with the old ONC, the maths started with adding up, some pople thought it was a joke, by week 5.6 we were on triganometry, i was glad of the steady flow inot it! dont be put of by going back to basics and starting there. out of all the people who dropped out my courses around 80% of them was because of the maths. It is built inot other subjects too
good luck all