Final year grade calculations post HND

How is a degree top up grade calculated if you undertook a HND (Higher National Diploma) instead of the first 2 years of a degree?

Myself and multiple friends studied a HND in music production at our local college and we are currently graduating in a music technology degree top up year at Portsmouth. We have been told our final grade can be calculated in 3 different methods which involves the 2 previous years of study in the degree programme. Grades delivered in HNDs are pass, merit and distinction and not based on a score out of 100.

Based on my performance on this year alone, I am looking to graduate with a 2:1. However, I completed my HND with my final grade as a distinction - could this influence my final grade in my degree or will my final grade be purely based on my performance this year?

The grey area is that I'm not sure if my performance on the HND is relevant to my final year calculation because it is recognised as a separate certification - my unit marks display the first two years of study as 120 credits but the grades are not applicable

I believe most universities go solely off the top up year, but if you look up their assessment handbook it should tell you