Dear NM,
I'm afraid I don't have a lot of advice to offer but wanted to reply as your last post made me so sad.
From what your are describing I think the time window for discussions with your H has closed. For the sake of your own future, but even more for your dc, I think you should see a solicitor ASAP. Go ask round here or in Legal matters section maybe someone can help. You need to see a lawyer to estimate what is due to you and t get on top of your financial situation. I assume your H bought the items for the new flat & paid the deposit from your joint account Please, please, please get in touch with your bank, get all the statements together & inform them of your situation at home. Maybe put a 'lock' on your account so not more than £300,-/day can be taken out without written consent from both of you. Definitely set up your own account & direct children's tax credit into it! Please don't stick your head in the sand, I'm afraid his mean-ness and moneypinching will get worse once he listens to OW and sees you and your dc as a liability.
Do you ave close friends in RL? Is your family nearby? Now is the time to ask them for full support. Are you close to your H's family? Could you get them to back you? As the mother of his children and their grandchildren, the least you deserve is respect. Don't let him treat you like this!!!
Is there a chance for you to fiddle some £££ away? Maybe write out some cash cheques for yourself from your joint account. It'll help you should need be.
I suggest you get your documents together ASAP: bank statements, insurance policies, house evaluation, pension, etc. Do not let him bully you. Keep quiet about it, just get it all together. Once you have a solicitor, give him all the docs.
I'd seriously consider naming (and shaming) the OW. It'll be on public record and some companies might shy away from hiring someone so disloyal and scheeming (sp?). This fact of his adulterous ways will be on public record should he ever try to tell lies to anyone & esp. your darling kids.
Be strong, smack him back! (Emotionally).
Hugs