Yes you are correct that the nitrates and phosphates in tap water feed algea.

Firstly, ponds nearly always turn green at this time of year as the sunlight intensifies and water temperatures rise. Getting the balance right between fish and plants is essential.

However, the thing that needs addressing is the UV. Allow 10 watts of UV per 1,000 gallons. The essential bit is UV maintenance. Have you changed the UV bulb lately?....they should be changed every February/March time as they lose effectiveness very quickly. I only ever run my UV from March to November.
Also, is the UV quartz glass sleeve within the UV clean ?....that can sometimes get a bit mucky and therefore reducing the amount of light treating the water.

A good clean of the pump might not go amiss too....clean any inline detritus filters out and make sure the impeller is clean and moves freely.

Obviously, water and electricity don't mix so use a circuit breaker and be careful.

I always put up garden umbrella's around my pond to stop it going green, but not ideal on a windy day as I have to go knocking on neighbours doors to retrieve them LOL, I'm thinking of having something going over my pond like a sail with a few small holes in it to let the wind go through, thats if it ever gets done LOL.

Haha LadyEssex. Of course, you could just increase the amount of surface cover you have with waterlillies or other floating plants. Aim for 2/3 of the surface to be covered. Marginal plants, oxygenators and lillies all use nitrates and phosphates etc as food. The more of these you have, the less food the algea has.

My husband used to put a bale of barley in the pond he looked after and that miraculously did the job -(you don't need a whole bale for a small pond).

A continually topping up with mains water will contribute to the problem. Many ponds will clear themselves once the excess fertiliser chemicals have been used up. Some people use barley straw as a treatment too but I haven't tried it
If the lilies have been repotted with ordinary compost that will contribute to the problem as well for the same reason.

I put 2 barley straw pouches in my pond (available in garden centres) and change them early spring and late summer, I do not have a pump and my water never goes green. Never remove the old ones till the new ones have been in the water for 4-6 weeks.
I agree that mains water will help to change the natural balance in the water and help it to go green also if not left to stand for 2-3 days before adding to the pond it will may kill the fish.
I use a water butt to top up my pond when its a bit low but I would never remove and refill water to control a problem as this will also up set the balance and the creatures that live in it.
Are there any plants in garden soil? that are draining into the pond as this will add to the imbalance.
An excess of fish food will also rot down and contribute to the problem all food not eaten in 5 mins should be removed.

I also put a garden umbrella over it when very hot sun is out to help keep the water cooler and shaded.
Have they got a water meter? as this regular 2 hour filling could be costly and a waste of water.

yes barley straw is the answer as it uses up the nutrients in the water.

Shouldn't need barley straw if the UV is working properly.

mine has been green for 5 weeks, yesterday when I came home it was clearing and this morning I can see all the way to the bottom.

if you leave it alone ie no water changes it will settle as the plants remove the nutrients from the water. that then deprives the microscopic algae and they die, leaving clear water. we stopped wasting money on a uv bulb in the filter years ago and just let mother nature do her thing.

Thank you everyone for your help. UV light has been changed, glass cleaned, oxygenators purchased plus some surface plants and a couple of barley pouches for the summer and we'll stop draining and refilling every week! Hopefully nature will now find its own balance.