formula to work out the value of a cell from another sheet based on a unique number

i have a spreadsheet with 3 sheets, if i start from sheet3 and work back,
sheet3 will contain a long list of postcodes and customer numbers.
sheet2 will contain the formulas to get the data from sheet1 but in sheet 1 some postcodes will be missing from time to time so what i want to be able to be able to do is get the postcodes from sheet3 by searching on the customer number then taking the postcode next to the customer number.
sheet1 will contain postcodes and customer numbers but some of the postcodes will be missing.

ive attached a example hopefully that will make a little more sense.