After banging my head against this problem from a different server, as WSL is not there yet for running as personal production services, I finally managed to hit the right keywords to google fu the answer. php-smbclient has been dropped from the officially repositories and so can only be installed via a PPA.
Installing using the following commands finally sorted it for me, credit to Markus (linked below for this solution)
echo "deb http://ppa.launchpad.net/ondrej/php/ubuntu $(lsb_release -cs) main" | tee /etc/apt/sources.list.d/php.list
apt-key adv --recv-keys --keyserver hkps://keyserver.ubuntu.com:443 4F4EA0AAE5267A6C
apt-get update && apt-get install php-smbclient
phpenmod smbclient
service apache2 restart https://markus-blog.de/index.php/2020/06/10/how-to-install-nextcloud-19-on-ubuntu-20-04-with-postgresql-12-php7-4-fpm-apache2-and-http-2/
