Do I need building regs to open up a chimney breast.

You don't need planning permission to alter a chimney breast but if it's a listed building you probably will but when you do the work make sure you use an adequate lintel as there is a lot of weight pushing down on it If the fireplace isn't being used then by far the safest method is to completely remove the chimney stack I can't comment on the freehold as we don't have leaseholds and freehold in Scotland

It will tell you if you in your property's legal pack, legal papers etc if you need to ask the permission of the Freeholder for works such as this. I would suggest you will do based on my experience of working with leaseholder V Freeholder projects. You will need to also check if your neighbour who's Chimney breast backs onto yours hasn't removed their breast as if they have the rules of removal change massively. You will need to use the correct thickness / size Steel plate, Gallow Brackets and fixings to ensure a good support of the Breast that remains above the room your removing it from. Building regulations will need to adhered to but check with your Council on what they require as all councils require different things from one another. Lastly, get someone that knows what they are doing to carry the work out. Its NOT a DIY style project.