Hi Harpurs,
in the Journal entry there should be a VAT box, with a dropdown. the Dropdown should contain an Exempt (0% Sales) option. I believe this comes as standard out of the box functionality. Choose that and then continue and save the entry.
If the tax is not there then go to the Taxes option, which shows accrued VAT and then top right use the "Add Tax" button. this will allow you to add the tax and then this should appear in your list to apply in the journal.
If not press the refresh button, the circle with arrows, and then try again,.
Hope this helps.
RoscoPc