Can I be forced to work full-time?

If you work part-time, and you've done so for a while (as long as it's in writing and it wasn't expressed to be a temporary change), you've got a clear breach of contract claim. You could even (if you were so minded) say that it's a fundamental breach and resign in response to any clear attempt to make you work full-time, claiming constructive unfair dismissal. It may also be indirect sex discrimination, since more women than men have child-care responsibilities - hence imposing a requirement on a woman to work full-time can amount to sex discrimination.

So, that's all well and good, and you do have legal protection - but the fact remains that I'm sure you don't want to have to go down the legal route. As you say, there is always a risk that if you complain, something dramatic may happen - and knowing that you have theoretical legal protection doesn't comfort you that much. Could you perhaps think about writing a very delicate letter reminding them of your hours, saying that you are prepared to be a bit flexible and have no objection to the odd call on your days off - but that the extent of the recent calls on your non-working time are becoming excessive. You can emphasise that you don't want to be awkward and are simply reminding them of the position. See where that gets you. As a next step, you could always issue a formal grievance, but you may feel that's escalating things too far in the first instance.

Best of luck to you - this behaviour on the part of the company is not fair, and you have every right to stick up for yourself.

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