"SoftKitty what I meant when I said his circumstances haven't changed is that they haven't changed recently. He has been working part-time for a few years, it's not only just happened."
Then what I said earlier still stands - you should be grateful to his wife for helping to support you your kids for the past few years since your ex went part time.
"If he had reduced the money when he lost his job then ok, but he didn't. I assumed they kept it up through guilt."
He didn't reduce the money because he sounds like a good man who wants to support his DD, despite not being allowed to see her. You are still on about him/them being guilty - I'm assuming because he left you for her - and I say again, you need to move on from this.
"I don't get why so many on here are telling me I should get a job - why shouldn't my ex go out and get a full time job then he could contribute more again?"
Because as you've clearly stated he's an older man claiming a pension and no doubt wants to enjoy his (partial) retirement and spend time with his kids. You know, the ones he's able to see. He has worked full time and now part time, and he's still contributing to your DDs upbringing. You, meanwhile, have not had a job for 16 years. Stop watching Jeremy Kyle and get applying for jobs.
"Like a few of you say it works both ways."
You said it.
"Their children are younger so they could get childcare tax credits I can't because my children are older so Id have to pay childcare from any wages."
You'll be entitled to working tax credits as you have kids at school. Look into it a bit more or go to the job centre for advice. But you won't will you, because you've got every excuse under the sun for why other people/the state should pay for your kids but you shouldn't have to work. It's time you started to take responsibility for your own finances, starting with getting a job.