technically - and i mean if you interpret the relevant legislation is an ultra pedantic way (but thats how the authorities do it with everything anyway) - cotton, coils, random plastic squonk bottles and yeah even tweezers and cases are likely to be "components" (as per Section 16(2) of the relevant legislation) of vaping products. and importation of "components" are equally banned as the the actual core products - atomizers, mods, juices themselves.
it doesnt help that alot of online retailers have to include an invoice/packing list with the product details (which might state - cotton for vaping, coil for vaping, ecigarette coil) depending on the shipper they use and where they are shipping from. so those will be red flags for customs.
plus - if you google some horror stories, you will find a few stories of people having such spare parts detained, only to have their places searched for the actual main products. so i wouldnt put it past customs to detain peripheral products in order to conduct a sting op on you for the actual main products as those will definitely attract liability even if you mount an argument that all these peripheral products shouldnt.
sorry for all the gloom and doom. what i've mentioned so far is the definitely the worst case scenario and an extremely pessimistic view. but from what i've seen so far (not in such cases of course, vapers are just the newest in a line of people getting whacked by our govt for "illegal products"), i tend to be very very paranoid and always erring on the side of caution.