I'm afraid I won't be much help to you with this one...I've dug out my notes and it looks like I resigned myself to the fact I couldn't get my answer to agree to theirs!
The sick pay element was fine - 7 days payable - (£88.45/5)*7 = £123.83
What i couldn't work out for the life of me was the salary she was paid for that month. I think I tried everything I could think of. But from my workings, she had 21 working days in May (Mon-Fri), 10 of which she was sick (3 qualifying, 7 paid). So I couldn't tie in the remaining 11 days that I thought she should have been paid a salary for. I would have done (£4,000/21 days)*11 days = £2,095.24. But maybe I was on the wrong path altogether!
I hope someone can enlighten you, I'd be interested to know how it was done! Although if everyone you've asked couldn't work it out either it might just stay a mystery!