Hi,
/p1:MTPODSalesLSUK/Record_Set/Data/MTD_Quantity[not(starts-with(.,"0"))] EX
/p1:MTPODSalesLSUK/Record_Set/Data/MTD_Quantity ≠ '0'
/p1:MTPODSalesLSUK/Record_Set/Data/MTD_Quantity ≠ "0"
Don't use apostrophes or double quotes when using NE or EQ. Just use 0 and it should work.
Regards,
Mark