I take OC's point's re the possible, indeed likely, hassle if a battery needs returning. Also, that, until you have a fault you cannot really know the quality of the supplying company.
Knowing this I purchased direct from Tayna about 6 months ago. Making a saving of about £35. Great service, and the battery is fine. So for the saving I was pleased to take the risk.
This is their policy on faulty batteries.
Faulty Items
Although faults with batteries are very rare, we do occasionally get items that are faulty after purchase. We aim to handle all faulty items returned to us as quickly as possible to avoid inconvenience to you. We strongly suggest checking that your battery is where the fault lies as in many cases it could be a faulty alternator or boot/glovebox light sticking on that is causing the indicated problems with the battery. As many as 50% of batteries returned to us for test are simply flat and need recharging!
After a faulty battery is confirmed we will pay to ship a replacement back to to you, but where no fault is found, unlike some companies we don't charge a test fee but we do have to charge carriage back to you at cost. Any non-faulty items not collected within 3 months will be disposed of.
John