This statement is from the Office of National Statistics following the 2021 census:

In Welwyn Hatfield, the population size has increased by 8.5%, from around 110,500 in 2011 to 119,900 in 2021. This is higher than the overall increase for England (6.6%), where the population grew by nearly 3.5 million to 56,489,800.

2021 Census
Hatfield        41,265
Welwyn Garden City        51,524
Surrounding District        27,011Note
Total     119,800
Note: in 2011, Welwyn had a population of 8,425; no data for 2021