Bread

@user86460, @kboy - you're feeding your starters with organic flour right? Strong white bread flour? Equal amounts of flour and water? Sorry if these seem obvious questions but organic does make a difference when it comes to the microbes available.
Where are you storing the starter, and are you giving it a good stir when you feed? It's not necessarily essential but the bubbles are carbon dioxide so if you get rid of those and manage to incorporate some air (and therefore oxygen) then the yeasts have more to work with.

You don't have to feed them much, I normally feed about 50g flour + 50g water before I use them but sometimes as little as 25g and sometimes as much as 100g if I need more. I've fed mine with organic wholemeal when I've run out of strong white in the past and it was fine but it was well established by this point.

I've baked two loaves and they taste great but they're not even an inch high, even though they've been proved in a banneton and everything. They just slowly collapse in the oven and end up like a baked jellyfish.

Sounds like your dough may be too wet, what proportions of flour to water are you using? I normally do roughly 2:1.
Also are you just letting the banneton do the shaping, or do you shape your loaf before putting it into the banneton?