scarer wrote:'ll probably get a tall larder fridge without a freezer compartment, then I can get either 4 cornies in it or one fermenting bin.
But until I actually get a fridge, what's going to give me the most noticeable improvement?
If you get the right one you can get 8

Cornii in it (I know cause I actually took 8 Cornii to Comet when I went looking

)
Temperature is the most important factor in brewing . .. Mash temp and Fermenting temp. Yeast is another factor, you should see a change using liquid to dried, but without temperature control its unlikely to be very noticable.
I would suggest using more grain and stopping collecting wort earlier, will give you a better quality wort, otherwise the little things you do like water treatment etc are really only minor