Check initail contract of employment with the staff.You do have one? Was it implicit within the T & C's that you provided trasnport to and from their place of work?
Qusetion (1) How long have they worked for you?
More than 13 weeks, and you have have to give them due notice of change of "accepted terms."
(2) What Health & Safety audits/provisions have you undertaken in respect fo one person, entering a building and working unsupervised on their own.
(3) Assume they (the staff) never contributed to the cost of getting themselves to work, what allowance, if any, have you offered to cover any increased costs to themselves.