It's a bit complicated, but as I understand the situation, when a tenant is evicted any goods that she abandons becomes the property of the landlord, but goods that she doesn't abandon and just leaves behind remain hers. That's for goods, stuff that isn't goods but is improvements to the plot is the landlord's.
So for example, paving slabs that have been laid, or a pond that has been constructed, because they're now effectively part of the plot, become the landlord's. I would expect that any plants in the ground are also the landlord's.
Stuff that isn't part of the plot but is just left on the plot, like a shed, or things in the shed, are goods. Goods that have been abandoned become the landlord's, but it's not easy to say what constitutes abandonment, because it all depends on the tenants intention and you can't necessarily tell that by the way the goods were left behind, and simply inferring that anything left behind has been abandoned doesn't work. Tenancy agreement will often contain a clause saying that anything left behind becomes the landlord's, but that's just words and it doesn't affect the position that goods that haven't been abandoned remain the property of the tenant.
The landlord becomes what's called an involuntary bailee for goods left behind by the tenant, and this imposes obligations on the landlord to protect and maintain the good - it's an obligation, though the landlord can charge for this service so isn't out-of-pocket. The landlord may dispose of the goods, but she must follow the procedure in the Torts (Interference with Goods) Act 1977, which I think means making every effort to send the tenant a notice of the landlord's intention to dispose of the goods. The tenant must be given reasonable notice to accept delivery or collect the goods, or to attend the sale if she can't accept delivery. The goods must be sold for value - so the landlord has to get what they're worth and not sell them on the cheap, and the proceeds of the sale must be given to the tenant - though the landlord may deduct her costs.
It's a tort (a civil matter, and the landlord can be sued for damages) for the landlord to allow the goods to deteriorate, or to dispose of the goods otherwise than in accordance with the Act. It's also possible that it's theft (a criminal matter).
As always, I could be wrong, but that should give you some pointers.