Excellent clues, thanks! I’m not inclined to install a Polish keyboard layout for my very occasional use, but you did give me a path to work this out.
Capturing it for others in the future:
Hold down the right hand Alt key (sometimes called the AltGr key) and using the keypad enter the four number character code.
For example:
AltGr 0233 = é
AltGr 0225 = á
You can find the code numbers at this page, or the table is reproduced below.
