As others have said, you first need to find out if the land is registered or not.
Given that this is just a random piece of grass you won't be able to search for it as, for example, No 23 XXX Avenue as it won't necessarily have an address.
What this means is that you will have to search what is called the "Index Map". You will have to send a plan of the area that identifies this piece of land to the Land Registry then they will let you know if it's registered or not. Details of how to do this are here:-
www.gov.uk/get-information-about-property-and-land/search-the-index-map
What you do next depends on whether it turns out the land is registered or unregistered.
If it's unregistered then, yes, you need to fence the land and it would probably help to show intent if you did open up an access via the coalshed.
It would probably help to keep some evidence of the date this was done.
You then wait 12 years and can then apply for the title of the land.
If the land is already registered things are a little bit more complicated.
First of all, you need to wait for 10 years. You then make an application to the Land Registry and they will contact the registered owners and let them know that you are making an application for adverse possession.
The registered owner then has 65 working days in which to object to your application.
If they don't object in time then, great, the land's yours.
If they do object then your application is rejected, but that's not quite the end of things.
The owner then has two years in which to start proceedings against you in court to get off their land.
So, if you either agree to vacate the land or they start court proceedings against you within two years then you're probably not going to get the land.
However, if at the end of those two years the owner hasn't started court proceedings then you're free to make another application and this time it can't be refused.
Although, please note that if the land belongs to the Crown or government departments etc then the time is actually 30 years