I've managed to concatenate 4 cells into 1 but they are custom format
(as they need to be a certain length) so many have leading 0's. The
cell displaying the joint information is missing out the 0's. How can I
get the concatenation to show all the numbers?

eg. 06263212103641 + 09940351822154315 + 0000000000461579 =
62632121036419940351822154315461579 but I want
06263212103641099403518221543150000000000461579