Forms :: Looking Up Data From A Table To Set Form Field Default

I have a basic invoicing setup, with a Form (Invoices) and subform (InvoiceDetails).When in the subform, i have a combo box to choose a Product Code (saved in table as PCode).I want now to auto fill in the NettPrice and (Product Description) PDesc fields in the subform row - by looking these up in the Products Table and entering the data into the relevant fields on the Subform. This lookup will be based on PCode.

I tried all sorts of methods and the one i favour, if i could get it to work, is setting up a Function then calling this function from the Default Value property of each field involved.So, for the Product Description field (PDesc), i created a Function as follows:

Function GetDesc () As String
GetDesc = DLookup ("[PDesc]", "[Products]", "[PCode] = " & Forms!InvoiceDetails!PCode)
End Function

Then i try to call by entering =GetDesc () into the Default Value property for the PDesc field.I seem to have a syntax problem with my function code.I know some of my values like NettPrice need not be fields on my Invoice Details table, but the prices change and I also need to be able to overwrite prices etc when typing invoice.