In recent years, part-timers have got a much better deal thanks to union action and European law. However, many of the workers who earn less than the annual earnings threshold for workplace pension auto-enrolment work part-time.
In general, it is now illegal for employers to prevent part-time workers from joining an occupational scheme and many part-time workers who were discriminated against in the past have also been able to build up 'back pension' if this applies to you and you have not taken action you have probably now missed the time limits.
If you are in a salary-related scheme, your benefits will be worked out on a pro rata basis (e.g. if you work half time, you would get half the pension that you would get if you were a full-time worker for that year). If like many people, you have years when you worked both full-time and part-time, you may want to ask your scheme administrator how you will be treated.