Lady T First a disclaimer- I'm not a employment lawyer so can't say what option is right for you but I am an employment rights adviser (well I was, now I manage them!) so I can say what your options are in these circumstances.
Now to the problem. You said that you had agreed these changes with your employer. What form did that take? Did they put anything in writing? If it was a firm agreement then effectively your contract was varied to say that you would be part-time & work from home occasionally. Obviously if they put this in writing (not necessarily giving you a revised contract but an email or letter confirming it) that is better as you have something to point to and back up your argument. If there is nothing in writing then that is harder but not the end of the world. An employment contract does not have to be written down to be legally binding. Once you both agreed to the changes they couldn't change their mind and revert back to the old one iyswim. They could give you notice that they will be ending your new contract and offering you the old one (full time, in the office) back. The minimum period for that would be a week for every full year of service up to a maximum of 12, or what notice period is in your contract (whichever is the greater) but they would have to have a pretty good reason to do that.
Obviously they will be trying to argue that it was never a firm agreement just that they would discuss it when you returned so think about what you have that backs up your version of events (emails, letters etc where you mentioned your agreement)
The flexible working laws are crap. If you can't sort this out then you could try a formal flexible working application but tbh if they say the business can't support it then there is very little you can do.
What you may have in your favour is the comment that they didn't think you would be back to work so didn't think about it. There is potentially a Sex Discrimination issue there which you could use as a bargaining tool. It is the kind of comment that indicates that they haven't given your request proper consideration because you are on Mat Leave. If you have already made a formal flex working request then this is what you could use to challenge their decision at tribunal as it suggests their decision is not actually based on business reasons.
So options open to you? My first stop would be your hotshot employment lawyer friend who ccan give you proper legal advice on your situation (rather than a random person from t'internet .) Then decide how to play it. If you had firm agreement and are confident about fighting it then you can claim that you have had a variation of contract imposed on you and make a claim for unfair dismissal (you can do this even while still working for them) Add Sex Discrim to the claim as well for good measure. This is a very hard-core option- as you said this can mess up your employment relationship with no guarentee that you will get something.
Softer options are to raise a grievance and appeal their decision- gather evidence to support your case for homeworking, why do your clients even have to know that is what you are doing if it is all by email and phone? You can go in as soft or as hard as you like. For a soft approach you could say something along the lines of "your comments about how you did not expect me back made me feel unappreciated and that I have been punished for taking maternity leave" If they have any brains in their head they will pick up on the Sex Discrim issue and try to resolve it in case you escalate it. If you do take it further you'd have to make it clearer that you regard it as SD but phrasing it that way at first means you can all pretend you aren't really saying it so they can face save and back track.
Option 3 is of course to ignore it, inwardly seethe and look for a new job.
Only you know what is best for you and ignoring it can sometimes be the best option for an individual so if you pick that then don't feel bad about it. Personally, I have done option 2 (issues combining breastfeeding and works request that I stay overnight for a training course) That approach was successful for me as it resolved the issue and there haven't been any long term impacts (I even got promoted afterwards!)
As for nursery- it isn't all doom and gloom. My 2 go 2 1/2 days a week. DS loves it (even complaining cos he had chickenpox and couldn't go in) and DD doesn't seem to have had a problem so far. When DS refused to sleep at nursery they did a form of co-sleeping where they snuggled up next to him. He now sleeps quite happily. I was so happy that they tried to deal with the issue in a similar way to how I would do it at home.
anyway dinner is ready so I better go, if you want any more info tomorrow then give me a shout. I'll be around during the day as DD now has chicken pox so having been back at work 4 days I have to be off again! Really though I think you should talk to your friend, just so you can make a proper informed decision about how to handle their rubbish behaviour. Good Luck