"I think you have received unfair dismissal from your job- have you received reasoning as to why you didn't pass probation? If not I'd be on the phone to ACAS." That's my first thought too! Not least because you have a limited time in which to start a claim for constructive dismissal or discrimination - and before anyone leaps on the '2 years employed' thing it doesn't apply for discrimination every worker is protected from that BUT I think op you only have 3 months to put in a claim so you need to get s shift on.
Secondly - WHAT were you thinking having TWO children with a man you don't live with who isn't properly financially supporting those children?! I can understand a contraceptive failure once but this sounds like it was planned! I'm sorry but when the father of that child isn't supporting them properly AND you already knew this before ttc no 2 - your 4th child - then I think you do need to justify. And as you were on good money why wasn't any saved?
Thirdly - as ever on benefits threads I recommend you speak to your local welfare rights officer (council based) and BE HONEST with them about your set up because as pp said its not just whether you're at the same address that makes you a couple in the eyes of the govt. If you're operating FINANCIALLY as a family in anyway they will classify you as a couple and take his finances into account too. They also tend to be better funded and have much better opening hours than CAB.
Fourthly - not your fault but he's an idiot for not getting proper cab drivers insurance which he clearly hasn't. I know several and one of my friends her dad has his own firm and has for many years. Most sensible cab drivers get the right insurance which includes a loan car for them to maintain their employment until the claim is fully settled. Costs more than usual ins but as its the tool of his trade he really should have done that. And as he hasn't he should be looking for temp work until he is back on the road, or even to temp do a cab share with another driver - a common arrangement among cabbies.
“"Living together as husband and wife/partners” is the test or standard they use. If you don’t, never have, and don’t intend to, youre perfectly entitled to claim as a single adult with children." If anyone's likely to be older I'd suggest it's you. That standard for benefit assessment went a few years ago (I suspect due to couples trying to claim they were Lp when they really weren't) I'm a benefits recipient myself and don't necessarily agree with this regulation but it is how it works now.
Also I've found CAB are usually well behind in benefits info I've not found them useful in that regard. They can be useful on employment law though and know which local lawyers cover which areas of law.
"You will be entitled to housing benefit from the council, child tax credits for your existing DC (just 2 of them if your third was born after April 2016), child benefit obvs and either JSA, possibly Income Support or possibly UC depending on area. Think both UC and JSA are done at the job centre now." Wrong to mislead the op. She MAY be entitled to a variety of benefits but without knowing her full circumstances (eg age, whether she owns or rents etc) we don't know enough. Also not knowing where she lives we can't know if it's legacy benefits (there's a few areas where this is still the case not many) UC for some claimants or fully live UC. We've no way of knowing and I'd caution op against giving strangers on web too much identifying info.
"I’m certain I was performing well. It’s a job I’ve done for over 20 years, although this is a smaller outfit" that does really sound like you've been the victim of sex/pregnancy discrimination. Your past record at other employers is also taken into account I believe plus smaller employers are more likely to do this as they can't bear costs of maternity rights (but then imo if that's the case they shouldn't be in business) but you NEED to get things moving on this to make a claim - not just as a way of getting suitable compensation/addressing it but because it also is a way of showing you're not "voluntarily unemployed" ie it's not your fault you've no job to dwp.
"but unfortunately I have no proof it was because of my pregnancy." Not how it works necessarily - the onus I believe would be on them given they headhunted you especially that you didn't fulfil the role adequately.
If you look at my posting history I'm the LEAST likely person to judge people on benefits/needing benefits poorly but in this situation you've not thought through your circumstances properly - either of you!
But whats done is done so for now:
1 acas - Re pregnancy discrimination, also lawyer ASAP
2 welfare rights office - they have good opening hours generally but could well be closing for Christmas soon. Take all financial paperwork and id
3 dp needs to pay a set regular amount as maintenance - you say he pays £X to cover certain costs for DC anyway, that needs to be formalised
4 job hunt - hopefully you'll be lucky and find a permanent job in your career but maybe if not at least a temp job for now? Dp needs to do same and possibly chase ins co if they're dragging heels?
5 cut down outgoings wherever possible - if you need help with this you could post on this thread, start another or look at or even post on mse site.