Thanks for the comments. I have tried communicating with a different unit and have had no trouble so I think I am either using the wrong address or the wrong data type
In my mind I have a vague memory that in the event of the slave not understanding the request or being unable to comply, it will repeat the request message back! Did I make this up or does this have some element of truth in it?
As you may guess, I am pretty new to this and trying to implement a metering solution on a very tight budget!