Hello and welcome! Visiting the UK was always on my bucket list!
Well, you’re not going to like what I have to say, but........ You have a very high fishload for that size pond. The fish are creating waste and the algae is there in response to that — trying to gobble up all the nitrogen waste. Your filter might be a good one (I don’t know much about filters), but if it can’t keep up with the bio load, you’re going to get algae!
Plants are the best thing for this — plants will compete with the algae for nutrients — but with that small of a pond, I’d worry that there isn’t enough room for plants and fish. Many of us have what is called a bog for filtration. Water from the pond flows through pea gravel — that is well planted — before it returns to the pond. This is a great way to add filtration for a small space. My “bog” is basically an extra pond connected to the main pond, but many, many people make container bogs — very cute and very effective!
