Whatever you choose make sure it is diluted as attempting to mix pure antifreeze into the tank will likely lead to having to flush out the whole system. It's a pain doimg this. ( Don't ask how I know )
Just a reminder that Blakes Lock will be closed for major refurbishment for almost 5 months from 30 October to 23 March. This means there will be no access between the Thames and the Kennet during this period.