Ah, great advice from all concerned. Thank you. I'm feeling safety in numbers here and it really helps. Like, really 
So: sling, check (I agree it's a life saver); slow cooker, check (well, looking at ordering one now); and have now moved bouncy chair upstairs for bath/bedtime routine. Baby not very keen on being put done at present but hopefully she'll get used to it? ??
Pleased also to hear that others are aaa reliant on TV and tablets to get through the difficult bits!
I do like to do bath-time with the eldest two - in together - most nights, not least to keep things in a predictable pre-bedtime routine but also because DD2 is going through a regression stage in terns of toilet habits (lots of soiled pants and an almost constant whiff of wee, if not poo
). I'm going to look at that baby bath seat idea of mootime's for when baby is a teeny bit older (worry that the eldest two will drown her with their splashing at the moment, although I guess she'll get used to it - poor soul!).
Vivi, you're so right, I need a constant reminder to lower my expectations. Seems obvious but this is by far the most valuable advice. Need reminding daily! I have this loud and persistent inner critic which keeps forgetting! 
Weetabix, I want your neighbour! She sounds ideal! We live in a very rural area and our closest young neighbour is 17/18 (and getting ready for A levels). Also, her school day seems to go on until 6pm, unfortunately. I often think she'd be perfect for the role in many ways (kids love her). The main thing would be to enable me to do reading/homework with eldest and spend a bit of one-on-one time with (currently quite needy) 3-year-old. But then another part of me thinks it'd be quite awkward to have someone there during the most stressful time of day & it may affect the general family dynamic. Plus, she'd see me being witchy mum! Not sure I can bear the embarrassment!!!! 
Anyway, thanks guys. I've enjoyed reading your experiences and coping strategies. Can't wait to be a little further along the line (in terms of coping confidence) but at the same time, like Lay suggests, I don't want to wish it all away as I'll never get this time back again.