Nitrates In Tap Water

Have any other aquarists noticed recent high levels of nitrates in Bristol tap water? I did a water change yesterday and noticed that the nitrate level had not been reduced. I checked the tap water and found that the nitrate level seemed to be well over 40ppm.

Just had a quick look it was Saturday the water was fixed... The store in question is The Aquatic Store if you have Facebook might be worth asking around on there

Legislation allows up to 50ppm in UK drinking water. Levels as high as you have are not uncommon, unfortunately.
The lowest you can get your tank nitrate by water changes alone is obviously the same as the tap water. Options for getting it lower than that include having a lot of plants, using RO water to dilute the nitrate concentration or by using a special filter to pre-treat the water.

One danger of relying on rain water - drought.

The test kits we have aren't terribly accurate. You need very expensive lab equipment for real accuracy. Your water company's website should have a water quality table somewhere, and nitrate is usually listed on that.

The Juwel nitrate sponge isn't reckoned to be very effective. Most people regard it as just another blue sponge in disguise.

Plants!
Mine goes from 40ppm in tap water to 10/20 in tank
It has quite a few plants, including a floating plant from my mum's pond which is starting to reproduce. See pic below.

You do need fast growing plants to be effective at reducing nitrates, and floating ones are probably the best as they are not limited by carbon dioxide in the water. Most floating plants will grow much longer roots when nitrate levels fall, I have amazon frogbit in one tank where the roots are now 40 cms long, easily reaching the bottom, and nitrate levels have plunged way below the level in tap water, though the tank is presently very lightly stocked. Many people might not like the look of the dangling roots, but to me they enhance the natural look I like to achieve, and the fish seem to appreciate them as well.