This is the output of sudo service dnsmasq status -l
dnsmasq.service - dnsmasq - A lightweight DHCP and caching DNS server
Loaded: loaded (/lib/systemd/system/dnsmasq.service; enabled)
Drop-In: /run/systemd/generator/dnsmasq.service.d
└─50-dnsmasq-$named.conf, 50-insserv.conf-$named.conf
Active: failed (Result: exit-code) since Sat 2018-07-07 22:41:49 BST; 13h ago
Process: 956 ExecStart=/etc/init.d/dnsmasq systemd-exec (code=exited, status=1/FAILURE)
Process: 859 ExecStartPre=/usr/sbin/dnsmasq --test (code=exited, status=0/SUCCESS)
Jul 07 22:41:47 NAS dnsmasq[859]: dnsmasq: syntax check OK.
Jul 07 22:41:49 NAS dnsmasq[956]: dnsmasq: junk found in command line
Jul 07 22:41:49 NAS systemd[1]: dnsmasq.service: control process exited, code=exited status=1
Jul 07 22:41:49 NAS systemd[1]: Failed to start dnsmasq - A lightweight DHCP and caching DNS server.
Jul 07 22:41:49 NAS systemd[1]: Unit dnsmasq.service entered failed state.
I have since tried uninstalling and re-installing pi-hole, then following this post: How do I revert to a previous version of Pi-hole? to do the downgrade, as I am still on Debian Jessie, and I still have the exact same issue.