Thank you all for your thoughts. You have, collectively helped me realise that I probably was being a bit unreasonable to be feeling put out - thank you to those who got where I was coming from, though. I have worked out quite why I was so irked by the interaction, which I will lay out further down (in case anyone is at all interested in the workings of my mind!).
To answer lots of you: I really wasn't expecting her to help me with my kids, or get off the bus, or any of the things I mentioned. I said what I would have done in the circumstances of someone else getting on with double the number of babies I had plus another child (and those who said I have all mobile children - yes, my one year olds are walking / cruising, but not confidently enough to do so on a bus!). So, as I said originally and in the title - my irritation came from her not acknowledging that she wasn't helping me, rather than from her actually not helping me.
I have realised that part of my irritation comes from the fact that she was breaking the rules of the buses - she shouldn't have been allowed to take on a non-foldable buggy (the idea is that if you are on the bus with a buggy, you have to have a foldable one so that you can get out of the way if a wheelchair user needs the space). Now, personally I think this is a stupid rule (as long as anyone who has that kind of buggy makes sure they do get off as soon as the space is needed for a wheelchair), and I have broken it in the past with a different double buggy, but I am very much a keep-by-the-rules person normally, so when I have done this I've been well aware that I need to get off if a wheelchair user comes on, and that I am getting away with it so to speak. And I kind of expected her to realise the same and be ready to get off if need be - so not that she should have got off for me, but that if she had been ready to get off she might have noticed me a bit more.... (it's complicated, in this little sleep-deprived head of mine)
The other thing, and the main thing I think, is that I'm actually irritated with myself - I am more and more aware how much of a 'people pleaser' I am - always looking out for how what I do might affect other people and feeling bad if I'm putting people out and so on. And I'm tiring myself out with doing this, and also getting irritable that other people don't look to do the same for me, in a 'it's not faiiiir' whiny sort of way... So thinking all this through, with all your thoughts to help, I am making resolutions to either help people out and have no expectations from others, or to be a bit more selfish myself and not assume that people are noticing and thinking the worst of me.
On some other random points that people have made: yes, I do think that two in a buggy plus a walking child 'trumps' one baby, in the sense that it is a lot easier to sit on a bus with one baby on your knee, than to do so with two and also keeping track of a walking child. But nonetheless, I didn't have an expectation of her getting off or anything else, I just wanted her to smile in my direction or something.
The post about my eyes BORING into the lady's back had me roffling a bit! I promise they weren't - I really didn't have much time to think about her while on the bus, it was only afterwards that I felt my irritation. So I don't think she knew - I even smiled at her as she walked past the bus on her way off, honest!
The post about calling us a circus - well, that's a bit how we feel, to me. My eldest tends to get cross very easily, I am not one of those calm, smooth sort of people, I have hair and sunglasses flying, and babies leaning out of the pram... And yes, if someone had come on with four children, especially if three of those had been non-walkers in a buggy I would have definitely got out of the way.
To those who think I should have expected the rest of the bus to help - a very kind lady did take one of my babies on her knee, so I didn't have to manage them both, and the bus driver was exceedingly patient and said not to hurry (I'm in Edinburgh, too, curly gal so know how unusual that is )but the rest of the passengers did sit and watch me with my unpacking and folding and juggling three children.... Which is fine - I honestly don't expect the world and his wife to take on my children for me. I guess I do have a crazy idealistic 'we're all mums together and we'll help each other out' sort of hope or idea, though....
This has been a bit of AIBU-as-therapy, so well done if you have got to the end of this!