Hi Stephen,
Happy New Year and all that. Thanks very much for your reply. I figured it out eventually. I had to create a delegated zone on our ISP so the flow was a little like this;
app1.originaldomain.com CNAME app1.toourispdomain.com
Then on our ISP i got them to create a delegated zone with the GTM as the NS server and on top of that get them to create another CNAME
app1.toourispdomain.com CNAME app1.mydelegatedzone.toourispdoamin.com
A little messy but it did the trick
Thanks again, now watch out for my next GTM question !
Cheers,
George