The fact that this may be your ex-husband is not relevant.
An employer cannot unreasonably vary the terms and conditions of work without your consent.
You went from 5 days 2 days with mutual consent and that has been the situation for some years.
He cannot now ask you to go back from 2 days to 5 days unless you agree or there was an early agreement that once you had brought the children up, you would revert to the 5 days.
If you ask for a contract, he has to let you have one. If he doesn’t, you can take him to tribunal to make him.
Can I clarify anything for you?
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If he sacked you for not going in Monday, you have a tribunal claim for unfair dismissal.
If you deduct your wages for not agreeing, and doesn’t pay was agreed that you would be paid, then you have a claim to bring in the tribunal or County Court for unlawful deduction from wages. Deduction also includes non-payment.
Remember you only have 3 months from dismissal to bring a claim in the employment tribunal and if you are just 1 day late, your claim will be thrown out so, if it does happen, don’t hang about.