I seen a few sites quoting a 90% reduction over various periods including this one from 2012 mentioning a figure of eight million at the turn of the previous century.
https://www.bbc.co.uk/news/uk-england-19568347
"A recent report by the Environment Agency said the UK's water vole population has dropped 90% since the late 1980s, making it the UK's fastest declining mammal.
Numbers fell from 1.2m to just 400,000 in 10 years - while 8 million are estimated to have lived in Britain's waterways at the beginning of the 20th Century."