How did you go today OP? It must be so hard with such a little one! My DC is older than yours (just turned 1) and I was pretty anxious even so...
Unfortunately the heat is not over yet so here are some ideas from me...
It will be really hard to cool the place off tonight. Look at the Met Office website for an hour by hour forecast where you are, and figure out at what time it will be cooler outside than inside. (For me this won't be until 1am!) The first waking after that time, open all the windows all the way. If you managed to get a fan, keep the fan on to help blow the hot indoor air out. Then as soon as it gets hotter outside than inside in the morning, close all the windows and curtains. For any east facing windows, close them as soon as you can after the sun comes up. And if you can somehow hang up extra sheets or towels or something to block more of the light, that might be worth a try.
Tomorrow is likely to be hotter than today for most of the country, plus you'll be starting the day off with a hotter flat. So be prepared for it to be even worse sobs Is there anywhere you could go for the day that's a bit cooler - family, friends, neighbours on the ground floor? I'm sure people will be willing to help with such a tiny baby.
Otherwise, do whatever you can to keep the little one (and yourself!) hydrated and cool. Your ideas (extra feeds, no clothes apart from nappy, cool baths) are all great. My DS and I spent over an hour playing together in a cool bath today! You could also occasionally mist her with water or sponge her down a little bit - be careful not to do too much and make her shiver.
If she does get poorly, the first signs of this will be either a raised temperature or dehydration (fewer wet nappies etc). Keep an eye on these things as well as if she seems well in herself. If you do see any worrying signs, ring 111, I'm sure you'd jump the queue with such a little one.
If you are breastfeeding, drink gallons of water, it is so easy to get dehydrated breastfeeding in a heatwave especially in the early weeks when your supply is still building up.
Above all count down the hours until Wednesday when the cool change is coming!
I really hope this helps, thinking of you (and all of us heatwave parents!)