That's a lot of good information, thanks. It's especially encouraging that you have an acre or so to play with.
1st time, I messed up & the chicks didnt survive, I took them off their mum too early
What happened? Why didn't they survive? Why did you take them away from the broody? What were the circumstances?
I leave my chicks with the broody hen until she weans them. I've had a couple of broody hens wean them at 3 weeks old (in warmer weather) and totally leave them on their own to make their way with the flock. They did fine but Mama had spent three weeks teaching the chicks to stay away from the other adults and teaching the other adults to leave her babies alone. I've had broody hens wait over two months to wean their chicks. There is no set age when she will wean them. With my set-up that has never been a problem. Sometimes people can have problems with this, especially if room is tight. The 1 acre sounds good but the coop may be more of an issue.
Been letting them all out for half an hour a day closely supervised.
I assume all means the broody with her chicks while the others are out ranging. Good.
When can they look after themselves ok, so mum can be let out
I assume you talking about removing the hen and leaving them locked in there? I spent a year in London back in the 1970's so i have a rough idea of what your weather is probably like. At 1 month old your chicks do not need the hen for warmth. They are perfectly capable of eating, drinking, and such on their own. If you are going to leave them locked in there until you decide to integrate them you can remove the broody at any time.
When can they start being allowed to forage outside & start getting a bit of grit
I let my broody hens raise the chicks and decide this. One of the first thing my broody hens do is take them to a spot where they can peck at the ground and get grit, as well as bits of food. This may be at 1 or 2 days old. You can probably buy chick grit at the feed store, they might have it, and give it to them now. You can put dirt in with them, they will get grit from that. Some people cut a small square of turf (grass, roots, and dirt and put it in the brooder. If they are allowed to forage outside they can get their own grit.
When can I safely offer them grain, should it be crushed grain at first?
What grain are you talking about? At a month old they should be able swallow any grain, none should be too big for them. As long as they have grit in their gizzard they can crush and grind it themselves at an extremely young age. That's what the gizzard and grit is for. There is nothing wrong with crushing or cracking it for them if you wish. A lot of people buy cracked maize (what we call corn in the US). If the grain is ground into flour of meal they do not need grit.