It's irrelevant that you're a tenant - a trespass is a trespass and you are entitled to remove anything from your property. I don't understand why previous posters have said you're not. Would you ring the council to get them to remove a trespasser sitting in your kitchen? No. Same with a fence.
The analogy above depends upon whether the trespasser in the kitchen just arrives one morning during the course of one's tenancy or is already there when you move in. In this specific case, the analogy is that the 'trespasser' is already present as a 'squatter' before the OP moved in. And looking at that solid and aged concrete post fence one can further expand the analogy that the squatter has his furniture and clothes in place and is found cooking breakfast on his stove for his family! Hence one cannot just throw him out. One would have to go back to the landlord and get the landlord to sort it out before moving in. The renter has no idea whether the squatter has any legal rights. This is the analogy with this fence.....it predates the OPs occupation.
We have no idea by how long it predates the tenancy; we have no idea if there is a dispute in progress with the council or at what point that dispute is currently at; we have no idea if the council have conceded that the fence has the right to or can remain. Hence to suggest that a tenant removes a concrete post fence that was in situ where it currently is when his tenancy began is advice that could result in arrest and why posters have advised that the fence should not be removed and that this matter needs to be left to the council.
I am amazed that posters are advising the OP to physically remove that concrete post fence without any formal authority to do so. Although if he takes that advice and removes the fence, the police and the Judge will have a good laugh when his defence is .....'Collaborate and Despair on Gardenlaw told me I could remove it'