Specifically I mean would they have said the whole, "You do not have to say anything but it may harm your defence if you do not mention when questioned something you later rely on in court, anything you do say may be given in evidence."
Or would there have been something else said?
All I can tell you is that the middle clause was only added recently, as you can see in this article. It was previously just "You do not have to say anything unless you wish to do so, but what you say may be given in evidence."
The right to silence in law was only formally enshrined in 1910, but I cannot find a source on when police began issuing the verbal warnings and what the phraseology was.
Well the right to silence was only fully codified with the Judges Rules of 1912. Before that Police forces had no set script to follow and had leeway to come up with their own. We can see in some fictional sources that the idea of the right to silence well and truely predates 1912. For example Sherlock Holmes gives a version of the right to silence in the Signs of Four which was written in 1890.
"Mr. Sholto, it is my duty to inform you that anything which you may say will be used against you. I arrest you in the Queen’s name as being concerned in the death of your brother.”