To all who have assisted, thank you. Unfortunately we have decided that we cannot use ISPConfig 3 as it does not provide a satisfactory platform for our environment. This is very disappointing for us as it showed a lot of promise initially, however the DNS issues are proving a little to complex and long-winded to resolve.
A suggestion: ISPConfig 2 worked great with BIND - ISPConfig 3 has some really great features and a very nice front-end. Although some very good reasons have been stated for going with MyDNS, especially security, migration, simplicity, etc. it just does not not have sufficient footprint in the market to gain our vote of confidence - everyone runs BIND. Anyway, that is just my penny's worth.