When you are logged in as sysadmin in single-user mode (if applicable), open Receipts, select View, Customise Journal, Columns.  Then uncheck the Discount columns if they are checked.  I believe this is the problem.  On the View menu, there is also a Restore Window option that might help redistribute column widths for you.

Done that and now got a Discount Available column but I don't seem to be able to enter anything in it.   I pulled something up with 4 invoices on it and each line has zeros in the discount column but I can't change it at all.

Discount Available tells you how much Simply thinks is available to take.  The Discount Taken column allows you to take it.  Type in an amount, hit tab and the rest will show up in the Amount Received column.  Look at the Journal Entry on the Report menu to make sure it is what you want.

I have a Discount Available column but not a Discount Taken column!  So how do I get the Discount Taken column?

Sorry, I only unchecked one of your columns from the previous mail, went back in and got the Discount Taken column now - many thanks!