Okay I've finally sorted it! I've made some instructions just in case anyone comes across this thread 
WAN
- Go to 'Network > Interfaces > WAN'
- Under 'Common Configuration' select the 'Advanced Settings' tab and uncheck ' Use DNS servers advertised by peer'
- In the box below enter the OpenDNS Servers '208.67.220.22'0 and '208.67.222.222'
- Click the 'Save' button
config interface 'wan'
option proto 'pppoe'
option username 'user@domain.com'
option password 'password'
option ipv6 'auto'
option ifname 'eth1'
option mtu '1492'
option peerdns '0'
option dns '208.67.222.222 208.67.220.220'
LAN
- Go to 'Network > Interfaces > LAN'
- Under 'DHCP Server' set 'DHCP-Options' to '6,208.67.222.222,208.67.220.220'
- Click the 'Save' button
config dhcp 'lan'
option interface 'lan'
option start '100'
option limit '150'
option leasetime '1h'
list dhcp_option '6,208.67.222.222,208.67.220.220'
DHCP & DNS
- Go to 'Network > DHCP and DNS'
- Under 'Server Settings' select the 'Resolv and Hosts Files' tab ensure the 'Ignore resolve file' is unchecked
- Click the 'Save' button
Firewall
- Go to 'Network > Firewall'
- Under the 'Port Forwards' tab enter 'Force DNS' under 'New port forward' section
- Set the 'Protocol to TCP+UDP
- Set 'External zone' to 'WAN' **
- Set 'External port' to '53'
- Set 'Internal zone' to 'lan' **
- Set 'Internal port' to '53'
- Click the 'Add' button
- Once it's added to the list open it back up by clicking the 'Edit' button
- Change the 'Source zone' from 'wan' to 'lan'
- Click the 'Save & Apply' button
** If you're unable to set the exact zones simply select anything as you can change it in step 9
config redirect
option enabled '1'
option target 'DNAT'
option dest 'lan'
option proto 'tcp udp'
option src_dport '53'
option dest_port '53'
option name 'Force DNS'
option src 'lan'
Your firewall rule you should like the following
Finally reboot your router by heading to 'System > Reboot'