D'oh, mattrom & malone - you are absolutely right. Fix that and it's straightforward.
I fell foul of the irritating feature that the crossword solver app has, where it skips over filled cells as you enter a solution. Because I already had acrosser for 7d when I entered it, the fifth letter I typed got entered in position 6 : the erroneous S. I find this consistentlly annoying as my muscle memory wants to type in the whole word! The Independent puzzle app has a settable option for this 'feature', but I can't find a way to turn it off on my Crossword Solver app :-(