iPad keyboard not showing symbols

Hi

I have an odd problem on my iPad running iOS 11.1.1. When using the onscreen keyboard if I press the "123" button, the keyboard changes to numbers on the top row, then various symbols on the next 2 rows (@#£&* etc...). All good so far. If I then click on the "#+=" button to see more symbols (I'm actually after the plus symbol), the top row remains unchanged (ie showing the numbers and backspace), and the other rows change to what they should show. On my iPhone (running the same version of iOS) when at this point the top row shows other symbols ([]{}#%^*+=).

Any idea how I fix this?

Thanks

Phil