But when I click on a link within the portfolio page I get redirected to my old website e.g. /portfolio/kickstart becomes www/strategyxdesign.co.uk/portfolio/kickstart
You’ve not updated the link in your config.toml: baseURL = "http://strategyxdesign.co.uk" and you’re using .Permalink instead of .RelPermalink. The combination of that is giving that error. Changing either one would do the trick, however I recommend changing both.
You don’t need to update the config. You can change your build command to hugo --minify --baseURL https://www.domain.tld/ and it’ll use that URL irrespective of the value set in config.