Thanks Colin, yes Im thinking I might transfer the filter sponges and fish in the new tank and most likely the sand (will have to add more sand due to the new tank being bigger).
Would it be necessary to get seachem stability or would the filter media I transfer be enough? In mid September I'm going on holiday for a week so I'm thinking if I set up the tank the week after from now and transfer the fish and filter sponges the new filter media would have a good colony of bacteria on it if left after a month.
Once established, the population of these bacteria in an aquarium will be in direct proportion to the amount of ammonia or nitrite respectively. Nitrifying bacteria require 12-32 hours to multiply, which they do by binary division [each bacterium divides into two bacteria].
Nitrosomonas multiply in less time (12+ hours) while
Nitrospira require more time (up to 32 hours). In a new aquarium, it can take up to eight weeks for the bacteria populations to reach a level capable of eliminating ammonia and nitrite. So without a source of ammonia, such as fish or decomposition of organic matter, the bacteria will go into a sort of suspended animation or hibernation state; they do not die, not unless something else affects them, though there is obviously a limit to their "hibernation."
You don't mention live plants or not; fast growing plants (especially floating) would basically eliminate any issues with "cycling." But without, using a bacterial supplement might not hurt, even if over-cautious. I have used Stability with good results.
Not sure I am correctly reading the fish adding sentence, so to be clear I would not add fish to this tank until you return, just to avoid any issues arising while you are away which could mean a tank of dead fish when you get back.