The Wayback Machine shows a link was archived at 05:43:36 GMT to-day.
https://web.archive.org/web/20200430054336/https:www.theguardian.com/world/2020/apr/30/hospital-chiefs-increasingly-frustrated-with-uk-testing-failures
From that archived link,
"A Guardian analysis of the data suggests that around 20% more tests are carried out each day than there are people tested. On Tuesday, that rose to a high of 37%. The reasons are unclear, but it is known that retests because of results that doctors disbelieve are not uncommon. In hospitals, that can be because doctors are convinced a patient has Covid-19 and do not believe a negative result, so get the test repeated.
Although the lab processing is very accurate, getting enough swab sample of the virus from the back of the mouth or nose can be difficult. PHE said that originally people with symptoms would have a swab test of the back of the mouth and another one of the nose, but now there is a move to obtaining just one sample. That might mean a greater likelihood of retesting."