The extra cover for up to £800 is an additional ‘benefit’ offered by the company if you met certain conditions, such as registering with them, therefore if you failed to meet the conditions you would not be entitled to that. Your rights against the manufacturer actually end here.
Instead, you must consider your statutory rights under the Sale of Goods Act 1979 and the Sale and Supply of Goods to Consumers Regulations 2002.
The law states that the goods must be of satisfactory quality, as described and fit for purpose. If they are not, you will have certain legal remedies against the seller. There is no protection against fair wear and tear, misuse or accidental damage, faults that were pointed out at the time of sale or if you change your mind and no longer want the goods.
You will not be able to take any action against the manufacturer as the contract and therefore your statutory rights were with the seller that sold you the lock.
If the goods are not as described, of satisfactory quality or fit for purpose, you have the following rights:
1. Reject the goods and request a refund - this is only possible if the rejection occurs within a 'reasonable time'. This period depends on the circumstances, although it is generally accepted to be within the first month after purchase, so must not be delayed.
2. Ask for a repair or replacement – if you are too late to reject the goods, you can ask the seller for a repair or replacement without causing any significant inconvenience.
It is also possible to claim for consequential losses as a result of the item not meeting the statutory requirements and this is where you may claim that the value of the bike was a consequential loss.
A useful rule is that if the goods are returned within the first 6 months after purchase, the law assumes that they did not conform to the statutory requirements at the time of sale. If the retailer disagrees, it is for them to prove that this was not the case. However, if the goods are returned more than 6 months after purchase, it would be down to the consumer to prove that the goods were faulty at the time of sale.
As you appear to be too late to reject the goods, you can still try and resolve this by contacting the seller and asking them for a repair or replacement, plus to pursue them for the consequential losses. You can quote the applicable laws and rules as mentioned above. If they appear reluctant to assist, write to them one more time, warn them that they have 7 days to comply and inform them that if they fail to meet their legal obligations, you will have no choice but to report them to Trading Standards and start legal proceedings to seek compensation for your losses. If they refuse to cooperate then you are really only looking at potential legal action to take the matter further.