Insist on a refund. Say it has in inherent defect and you are entitled to a full refund under the Sale of Goods Act 1974 which states goods must be:
Fit for the purpose (it isn't)
As described
Of satisfactory quality (it isn't).
Stress you have been more than reasonable in accepting repairs twice but you have lost confidence in the product and it is still not fit for the purpose.
Be firm. Tell them you will not hesitate to make a claim in the Small Claims' Court (easily done online, and cheap) if you do not get all your money back.