I agree with Zippi ..... irrelevant of the family situation, there is only so much money to go round. End of. Your DH is paying what he should as defined by the CO, past experience has shown that when he was able to do so, he actually increased this sum for as long as it was sustainable, plus he gave the eldest an allowance.
He is on a pension FFS !
Sorry - if these kids refuse to help themselves by working then they have no right to moan and pressurise their dad. It's ridiculous to say that weekends are for going out and having fun. Whatever happened to working during the day, and going out at night ..... or working weekday evenings ..... or going out and having fun on a weekday ? Any ex student knows that you are not committed to study each and every day during the week ..... there's generally lots of "spare time" slots between lectures, not all of which are taken up with reading etc. and when I studied, I had at least 1 whole clear day each year of the course.
It's a sad fact of life that many parents cannot afford to fund their kids through Uni .... I have a 16 yr old (+ 2 younger skids) and a toddler, and I worry terribly about my oldest because I know I won't be able to help him. I hate the thought of him starting life with a load of debt but that will be his choice if he decides to go to Uni, minimised hopefully by any p/t work he can pick up. There are thousands of families in exactly the same situation you describe ...... very rarely is it the case that parents refuse to help their student kids through petty spite or meanness ..... most of them are eaten up with "guilt" at being unable to do so, since fees were introduced and grants withdrawn.
It's very simplistic - and, IMO, very immature to "blame" the new baby for the lack of funds from dad. These kids are quite capable of helping themselves and on the cusp of adulthood, (though I disagree with the withdrawal of govt funding for higher education) I don't actually think that learning independence, learning how to budget and finding out that you don't always get what you want is such a bad thing.
I know it's easy to say but I think you and your DH have to stop feeling guilty. It's clear from what you've written that you do care very much and I'm sure that were you to have a bit of good fortune, you'd use it to help his kids. If so .... then you have nothing to reproach yourselves for.