Part time workers should be treated in the same way as their full time equivalents and should be given the same rights and benefits (albeit on a pro-rata basis). Employers can only treat part-timers differently if there is an "objective justification".
A segment of part-time working, job sharing is becoming increasingly popular. When you employ two or more people on such a basis, the employment contract should pay special attention to the provision of mutual cover including hours to be worked, time off and holidays (including bank holidays). This is in addition to standard employment contract terms.