What does your capability or performance policy state? You need to look at that (and any solicitor will expect it). Also email whoever you had the meeting with and ask for the minutes.
it wasn’t a protected conversation unless they or you asked specifically for it. Protected conversations are specifically for having open conversations whereby the information cannot be used if it goes to tribunal. Both sides have to agree to such a meeting.
as you have been there for more than 2 years you do have your employment rights. So they can’t just sack you. Of course any organisation can manage you out and make it look legitimate, it happens all the time. If there is a clear link between coming back and being a mother and this process then you can claim discrimination but you have to have evidence, it’s not a case if you ‘think’ it is linked. They could have said no to 4 days and you should stop working the 5th day.
have you notes of 1:2:1’s (do you have them regularly)? Any capability is against your JD or any implicit changes that have been made over time (so if you started doing something additional lot in your contract it is implied that you accept doing that).
tribunals are hideous and if you were in a union they only take these cases through if it is really clear you will win. Have a look at ACAS’s website yourself as it will have helpful information.
but it is important to get what they said to you in writing and the capability policy. They cannot force you to resign but if they have. Policy they need to follow it or at least the law relating to dismissal.
settlements happen everyday. Figure out what you would get if you were made redundant plus notice period and this is generally what would be deemed a reasonable amount, unless they have clearly behaved badly, but even then there are formulas to calculate payouts.
for the future John a union as they would have probably felt with this informally where they could.
also start looking for another job because if they want you out they will make you life difficult