Oh dear, this is going to make me sound so MEGA boring...
In my life before lashes I worked in payroll :zzz:
Worked for a company doing all the government payrolls, paying MP's, doctors etc. And when they leave they get paid on their normal payday. Also worked on the American Express payroll, and once again when they leave they get paid on their normal payday, not on the day they leave.
I can't say 100% if what your ex employee saying is right, but if the big companies 'get away' with doing it every month, then I don't see the problem. But you can always double check with HMRC. Their employer helpline no is 08457 143 143, but your accountant will know.
That aside, if it was me I'd pay her on her normal payday. Why would you want to help someone to get thier equipment so they can poach your clients? Business is busines after all.
Besides, if she wanted to sue you, you would have paid her by then!
Yolanda x