We had one when we moved into our house. We asked our builders to demolish it as it was not usable. They wouldn’t give a price and instead would only do it on a daily rate as they said impossible to estimate. . It was v difficult to demolish as it had odd metal everywhere. We stopped doing it when the cost became too much and built a small wall round it. A few years later we decided to finish the job. My husband hired one of those mechanical breakers and it took him another couple of weeks. At least one machine burnt out because of the effort.
My advice would be to get a labourer or two to demolish for you and remove rubble etc to a skip. Make sure you set aside enough money to pay as It’s not a cheap job but worth the extra space when you get it back.