Thank you for that - though I think they don't pay until the following April after leaving university etc., and as you say, only if their earnings are over the threshold. Just in case anyone else is interested, this is what is says on HMRC page:
Students in higher education in the UK can get loans to help with their expenses while they're studying. The Student Loans Company makes the loans but HM Revenue & Customs (HMRC) collects the repayments.
Students start repaying their loan once they've left higher education and their income is more than a certain amount - the threshold. If their income is high enough, their repayments start in the April after they leave higher education.
The income threshold for starting to make repayments is £15,000 a year (£1,250 a month or £288 a week). Once an employee's income goes over the threshold, you deduct 9 per cent of their income that's over the threshold towards repaying their loan.
HMRC collects the repayments along with your employer PAYE (Pay As You Earn) deductions.
Thanks again for such a speedy response,
Lyn