Who would enforce it (it can’t just be a random neighbour)? They’d have to be so annoyed as to take you to court, spend a ton of money and maybe not win at the end of it. Plus if the landowners changed through the years the new one probably isn’t even aware (download the register extract from the land registery – some have convenants on it, and some don’t – if they don’t they’d need the original documents, which they’re unlikely to have/read/remembered).
I believe you could also take out an indemnity policy to cover it (tho if you do, don’t approach who you think are the owners without checking the policy wording as it might invalidate it).
Feel free to ignore the above as i’m certainly not a solicitor!
You will need planning tho if it’s infront of the wall of the principal elevation.
https://www.planningportal.co.uk/info/200130/common_projects/43/outbuildings