I own 2 knock off 900mah ego batteries and they are by far the best ones I have. My genuine Joyetech 650 mah blew up on the charger and was younger than my knock offs
You need to be extremely careful with batteries. I'm not talking about the media hype, but just a few words of caution...
The nature of electronic cigarettes is that you use them during the day and charge them during the night - while you are asleep.
The batteries and chargers generally need to be matched such that the correct charge is being applied to the battery...too little and the battery wont charge quick enough - too much and the battery can overheat and then you can run into problems.
Most high end equipment, including the tank batteries that you refer to have expensive batteries with control circuits to ensure that the correct charge is applied to the battery. Cheaper batteries may cut this corner and avoid the circuitry.
Avoiding the defences on a product that could easily catch fire while you are asleep, is probably not worth saving a few pounds on.
So, if you are going to import from China, buy directly from the manufacturers, not anyone that claims to be a Chinese distributor.
All suppliers in the UK buy in bulk directly from the manufacturers. they may add a massive markup to things like batteries so that they can sell you cheaper liquid or cartridges.
It isn't hard to shop around once you know the part number for what you are looking for, and when it gets to importing batteries my advice would just be - don't do it, buy from someone reputable.
(I don't sell batteries and have no commercial interest in what I have posted)