The Student Room Group
an 'advanced diploma' has 300 credits 120 at level 4 / F 120 at level I / 5 and 60 at level H/6 ...

a DipHE has no level H/6 credit

the grand upshot is that to 'top -up' from Adv dip you need 20 credits of modules ( so 1 or 2 depending on whether the HEI is says one module is worth 10 or 20) and the 40 credits that your dissertation is worth ...