Do I just leave them in permanently, or test my water again in order to decide when to take the shells out?
Permanently. If you take them out, the GH/KH will go back to what it was originally after a while.
I think I want to make a 'dutch style' aquarium, which if I understand it relies more on the plant life
:bounce: Yay! Haven't seen one of these done in ages, can't wait.
1) I should make my first layer 'First layer pure layerite' and then put my gravel or whatever over the top. Is this what you would use as a base? I want something decent, without going for the flashest out there. Can anyone recommend a product? I'm also confused as it said to wash it on the packet, yet I read elsewhere not to wash it, in order to keep the nutrients. After, I have laid down this layer, do I then add say a nice sand/thin gravel? How thick for each layer?
No, I wouldn't use it because I can't afford it. If I could then I'd give it a go, only heard good things about it. I just use regular gravel or sand and my plants do great. Total substrate height depends on which plants you want to keep, some plants require a deep substrate to hold their roots down. Normally it can be 3-5cm at the front sloping to 5-7cm at the back.
2) She said I could use 'root tabs' - necessary?
I've used JBL Fert Balls and these had no ill effects. In saying that, they also showed me no noticeable benifits either.
3) She said I could use API leaf zone? Necessary? Other similar products?
No harm in using it although I'd go down the PMDD route instead.
4) and I should have T5 light bulbs? Don't even know what I currently have. I can't see the wattage due to where the labelling is on the bulb and I am unsure how to detach the bulb in order to investigate.
I don't have T5 bulbs, again, too expensive but my T8's and CFL do fine. Not sure what kind of light you have though.