Are you using Internet Explorer?
We are aware that some people are struggling to login. This is due to a recent update with internet explorer and you will need to update your browser to the most recent one. This can be done at https://support.microsoft.com/en-gb/help/4056568/cumulative-security-update-for-internet-explorer
Alternatively, try https://achieveinscience.co.uk/wp-login.php or use an alternative browser, such as Google Chrome or Safari.