I have been keeping Grandis for a little while and this is what I have found to work. Anybody else, please contribute.
What is the best substrate to use? I am using newspaper at the moment, is that ok to use long term?
Newspaper is fine, it is not very pretty, but it is fine. A steril potting soil(super soil) or an orchid bark could work too. Something that can retain a good amount of moisture.
He is currently in a 1ft x 1ft x 2ft tall Vivarium, just for quarantine purposes, what sort of size will he need long term?
I would say 3ft high, by 2ft long, by 2ft deep. They are large geckos and can easily get 12".
Should he be getting offered fruit in his diet?
You can offer baby foods(such as banana, apricot, peach, etc) and the Meal Relacement Powder called Day Gecko Diet. I reccomend variety, see what he/she likes. You can also puree fruits for them, they love that.
The shop that I bought him from, when he was being taken from the tank, in to the container to take home, the tip of his tail fell off, I know that geckos can do this no problem, do i need to watch for anything, i.e. infections?
Obiously the move home was a streesful one. I would'nt worry about it too much. Watch the tip of the tail and if it starts to get pink and puffy consider seeing a vet.
Finally, with their skin being so delicate, how do i go about safely transferring him from his current tank to his new one when the time comes?
Use a small plastic container that can fit over him, and then place a piece of paper under the gecko and the container. They do not tolerate handleing very well.

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Cheers,
Michael Starkey
Arboreal Addictions