I am putting extra insulation in my loft, the current insulation comes up the the top level of the joists. I have enough fiberglass insulation batts to give an extra 10" layer for the entire loft. Will it be a problem ( building regs/fire /damp ) if I cover cables/forced air heat runs/water pipes?

Not knowing where you live I'd say just call your building inspector and ask that question. Codes and regs vary from plkace to place so checking with the "enforcer" is the safest.