My guess is that you are getting too much nutrients into your pond (nitrates), and coupled with the natural light these are stimulating the growth of algae. In turn the algae soup is providing food for the larvae of some flying insects: they could be mosquito larvae but there are many other possible species.
In one sense, you *have* created a wildlife pond, and what you are witnessing is the slow colonisation of the space by wildlife, albeit not the wildlife you were hoping for. One solution is to leave things alone and be patient to see what else moves into this little ecosystem. However, anything bigger than insects is unlikely. As the previous poster said, the lip will be too difficult for frogs to negotiate (unless you've sunk the barrel).
Ok, that aside, how can you "fix" things, and try to get clearer water?
- Shade the pond from the sun or move it, giving less encouragement to the algae.
- Avoid organic matter falling in as much as possible, to reduce the build up of excess nutrients.
- Suck out the excess nutrients and outcompete the algae by adding plenty of fast-growing plants such as canadian pondweed (
Elodea - Wikipedia, the free encyclopedia) or duckweed (
Lemnaoideae - Wikipedia, the free encyclopedia). Neither of these is native to Britain, so this is not ideal from the perspective of your "wildlife" theme, and you should take care they do not escape into waterways. Even better take SMALL amounts of waterweed from a nearby wild pond - those that are successful will reproduce endlessly on their own so you don't need much.
- You could also add plants (from a good aquatic or garden centre) which grow with their leaves above water and their roots in soil within the water (e.g. irises), or lily pads which help to shade the water - however you may not really have enough room for these. However, these can extract a lot more nutrients due to their aerial leaves.
- I would suggest removing the fountain. Moving the water like this reduces the amount of dissolved CO2 in your water; low CO2 levels prevent plant growth, which you need to suck out the nutrients and outcompete the algae.
- Make sure you only change a maximum of 1/3rd of the water at a time, and only add water that has been left to stand in a bucket for at least 24hrs (to allow the chlorine to escape; unfortunately won't help with chloramine which many water suppliers now use). Ideally you want to change this amount of water infrequently as it upsets the balance, but at the beginning you could try doing it weekly until the plants get established.
- If you want a more "high tec" solution, you could look at adding a filter (which removes solids and also acts as a home for various helpful bacteria) and/or a UV system (which effectively sterilises the water as its pumped past an intense UV bulb), but this isn't in keeping with my idea of what a wildlife pond means.
Re: Getting bored of my pond going stagnant! 
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