Well, just by way of an update, the ammonia levels are gradually reducing, and nitrites are at significant levels. I've done four 50% water changes since starting to try and keep levels of both lower, so I'm hoping the nitrites will be on the way down soon also.
Incidentally I haven't even opened the bottle of Dr Tims Ammonia yet - all the ammonia is being, and continues to be, leached into the tank by the Tropica Soil Powder substrate!
Thanks for the update.
What are your present ammonia and nitrite levels? Which bacterial supplement, if any, did you use? I'm not familiar with the Tropica Soil Powder substrate material. I guess the statement "We recommend that you change 25-50% of the water min. twice a week during the first 4 weeks after establishing the aquarium" is an indication for the potential leaching of ammonia.