Km
Hello June, welcome to the forum.
The barrel itself has to have liquid in it to be watertight as the wood is porous. The wood absorbs some of whatever liquid it holds expands and becomes watertight.
This brings me to a point you mentioned "oily film" in one of your posts. This is either from something was stored in the barrel or that it was treated with, which could very well be toxic to plants and fish. So I would definitely line the barrel. I am in the states so I have no Idea were you would find a liner In Britain, but I would try to locate a store that carries pond supplies or sells barrels as planter. Odds are if they sell barrels intended to be used as planters or ponds they would carry liners.
Also if you want fish in your barrel I would use mosquito fish. They stay small and would be a better fit for the barrel. I put goldfish in my barrel a year ago. They grew to large for the barrel in just a few months, and I had to build a pond for them. In the end that mistake got me into the hobby, so a happy mistake.
Hope this helps, cheers.