Just to be clear about DDA
If your mental illness has a substantial, adverse and long-term effect on your ability to carry out normal day-to-day activities, it is likely you are covered by the Disability Discrimination Act. The act also covers people who had a disability in the past.
The act doesn't impairments that are covered, but considers the effects of an impairment on a person.
For example, someone with a mild form of depression with only minor effects may not be covered, while someone with severe depression with substantial effects on their daily life is likely to be considered as having a disability under the act.
I would have thought you would be unlikely to be given a diagnosis of anxiety if it was just situational and considered very short-term and as Flash said, it would need to be long term, ongoing to be covered. Sorry if what I wrote was inaccurate, it is just that many people do not know that there is protection for mental health as well as physical conditions.
I will repeat what I think, that op should see Occ Health, request PT contract, and return asap, using A/L to achieve PT hours and if unable to cope with PT, do the decent thing and resign the post so they can recruit and fill it asap. Full time working after maternity leave is not for everyone and if the idea is making OP ill, I would suggest it is not the right option, but the NHS is not a cash cow but often seen as such by employees as it is so generous with sick leave etc.