You are in the UK which creates a language barrier. You have two types of grit while we have one.
Chickens do not have teeth to chew their food but they eat things that need to be ground up. Things like grass, grains, and some creepy crawlies. They eat small pebbles and use those as teeth to grind that stuff up in their gizzard. If they have access to the ground they will peck at the ground and find their own grit, they can pretty much use any rock they find if it is the right size. You call this insoluble grit. We just call it grit. You should be able to find it at the feed store. That will be granite, it is really hard and lasts a while. They sieve the waste at a granite quarry to get the right size so it is plentiful and pretty inexpensive.
The egg shells provide calcium for egg layers to use to make their own egg shells. People often use oyster shell. We just call it a calcium supplement or oyster shell, you can call that soluble grit. The digestive juiced causes it to dissolve so their bodies can absorb it, that makes it not suitable to use as insoluble grit. They do not need and should not have soluble grit until they are ready to lay. Too much calcium when growing can be hard on them.
They do not require insoluble grit to eat chicken feed that has already been ground up. like mash or crumble. Pellets either but pellets are too big for baby chicks. They do not need grit for scrambled eggs or egg yolks. Some cooked vegetables would be OK, like cooked carrots.
But some people feed a chicken feed that has not been ground up, you can still see the grains. The probably need insoluble grit with raw carrots as they are pretty hard. I like to feed mine insoluble grit at a very young age regardless of what they are eating. That way they are ready for anything.
Chickens do not have teeth to chew their food but they eat things that need to be ground up. Things like grass, grains, and some creepy crawlies. They eat small pebbles and use those as teeth to grind that stuff up in their gizzard. If they have access to the ground they will peck at the ground and find their own grit, they can pretty much use any rock they find if it is the right size. You call this insoluble grit. We just call it grit. You should be able to find it at the feed store. That will be granite, it is really hard and lasts a while. They sieve the waste at a granite quarry to get the right size so it is plentiful and pretty inexpensive.
The egg shells provide calcium for egg layers to use to make their own egg shells. People often use oyster shell. We just call it a calcium supplement or oyster shell, you can call that soluble grit. The digestive juiced causes it to dissolve so their bodies can absorb it, that makes it not suitable to use as insoluble grit. They do not need and should not have soluble grit until they are ready to lay. Too much calcium when growing can be hard on them.
They do not require insoluble grit to eat chicken feed that has already been ground up. like mash or crumble. Pellets either but pellets are too big for baby chicks. They do not need grit for scrambled eggs or egg yolks. Some cooked vegetables would be OK, like cooked carrots.
But some people feed a chicken feed that has not been ground up, you can still see the grains. The probably need insoluble grit with raw carrots as they are pretty hard. I like to feed mine insoluble grit at a very young age regardless of what they are eating. That way they are ready for anything.
very happy 
