Em, do you have a copy of a contract of employment?

Did they give you any written information with regard to your maternity leave entitlements?

Legally you are eligible to return to work to a "like or somilar" role. So even if your old job has ceased to exist, they MUST offer you a similar level of role on your old salary (or pro-rated for part time)

In terms of the business being sold, your contract of employment is with the organisation/buisness, NOT with the individual owners, so the new owners MUST provide you with a similar role to return to.

HTH