The 23-year-old from Southend, who in her heyday has won medals at the Olympic, World, European and Commonwealth Games, was 'surprised and delighted' with the time she achieved in gaining a significant victory.
In fact, it was around eight seconds faster than she had anticipated. She had been aiming to finish the distance in under 4min 20sec but raced home, more than a second ahead of the second- placed swimmer, in 4:12.28.
Swansea's Victoria Hale, the runner-up to Hardcastle, found some compensation by setting her third Welsh record of the championships with a time of 4:13.46.
Earlier in the day, Hale had broken the Welsh mark for the 200m backstroke. On Thursday she had beaten the previous record for the 400m medley.
Martin Harris twice lowered his British record for the 100m backstroke. In the heats, his time of 54.95 gave him pole position for the final and he took advantage of the opportunity in magnificent style, powering his way to victory in 54.60.
Mark Foster was another to show record-breaking form, capturing his second national title in as many days by winning the 50m freestyle in 21.87. That was 0.12sec inside the previous British and Commonwealth best, which he himself set.Reuse content