Argh! big hugs to you, and I thought I was momentarily stressed earlier, you've put my life back in perspective...
Work cannot just get rid of you. First read your contract and ascertain exactly what they are trying to do to you and where that relates to your contract. As you've been off on maternity they certainly can't 'do' you for that, and to get rid of you they certainly will need to do the 1 verbal, 2 written warnings line, unless they can get you on gross missconduct, which I doubt very much.
As stated previously if there is a union about get them involved, and make sure either way, that you take in a diary and write down every little incident, date and time that you have any bother at work (be assured they'll be doing the same) www.adviceguide.org.uk will take you to the CAB online for info, or there's DTi.gov, or tiger.gov.uk (I think off the top of my head to help) There is also workingfamilies.org.uk who may have good advice?
Sorry to hear the bad news about DH, lets hope it gets sorted soon, a friend of mine has beaten the big C twice now (and is still smoking?! Men eh!) so go gettem girl,
As for advice re your mother, I'm not at all qualified to help, but I agree with the letter idea, It may help, and for me personally, writing helps me feel better, even if it does nothing for the person on the receiving end, at least you tried, you cannot be responsible for someone elses past, even if it is your mother, that's for her to deal with (harsh but true) you owe it to yourself and your family to look to the future, forward and positive.
a ditty for you
Life, the passing of days, the parting of ways, tears, laughter, anger, sadness, in fact you could say its all total MADNESS