Hi Christoph et al

Firstly thank you for your response.

1. Services -> DNS forwarder -> enable

Confirmed.  Also ticked "Register DHCP leases in DNS forwarder" which I presume is required.

2. after: "You may enter records that override the results from the forwarders below."
add the internal IPs with the used FQNs

What are FQN's?  I have added a couple of domain names and the LAN IP which the server is on. 
domain names entered in the format 
domainname.com without the prepended www.

3. create a rule "allow DNS from inside to internal IP of the M0n0

I have created a LAN firewall rule from Any LAN, port 80 to the LAN IP of monowall (also tried LAN
IP of the server)

4. force the internal Clients to use the internal IP of the M0n0 as primary DNS

I am not sure how to do this.  Monowall is serving DHCP addresses to the LAN, my understanding that
includes Monowall as the gateway 
and the DNS server?

I'd be most grateful of further assistance.

Kind regards
David Hingston