Hi Angelos, HMRC finally gave me a solution. It is extremely unintuitive, as they admitted, and I wasn't the only person to call them about it.
Here are the notes I made at the time, hope it helps.
At the bottom of the 'Foreign income types' page, do NOT press "Save and continue".
Instead, press "Add" (totally non-intuitive), then the same page is re-displayed with
blank fields, but also with the foreign employment at the bottom.
Select the radio button, then click on "Save and continue".
Now a different page is displayed, where the following can be entered:
- Taxed UK interest etc
- Untaxed UK interest etc
- Untaxed foreign interest (up to £2000)