Hello
Don’t apologise - it’s been really helpful and it’s really appreciated. I hope my message didn’t sound as if I was ungrateful - that wasn’t the case at all - I just didn’t think I’d been clear about my issue in the first place so just wanted to mention where I was having most issues 
The array list looks really helpful - I’ll take a look at this - thank you.
I did do a bit of mapping but my numbers were all pretty low so I started fiddling about with my very messy formula
Thanks again
Becky
p.s. I’ll post again once I’ve looked properly at the array list