Right, I've got this sorted now. Im having the JBL aquabasis plus on the bottom. My tank is 80cm x 45cm and I need 2cm thickness that gives 80x45x2 = 7200/1000 = 7.2L . I have 12.5L so Im gonna use the 5L bag and a 2.5L bag and then smile nicely at my LFS to swap the two 2.5L spare bags I have for something else. (No problem, my local LFS manager is a fantastic guy). Thats £20 back in my pocket.
PLUS I have just bought 3 bags of Tesco cat litter. Thats gonna be my topping substrate. Cost, <£10 instead of >£40 for the Colombo florabase. Yippee..... Can't wait to get started now.
A BIG THANKS to everyone who has helped out on this post. PLEASE feel free to follow my 1st ever planted tank journal. Can anyone tell me how to put a link to it in my signature ? I used to be in IT support but it all escapes me nowadays lol.