It sounds as if the replacement keyboard has been incorrectly fitted, although short of seeing it I can't be 100% sure.
You're in a difficult position here, because the company has said it can find no fault with the keyboard - it's your word against theirs. I very much doubt that you're going to receive a response to your letter of four weeks ago, and if you are determined to get satisfaction on this you're going to have to take some form of action.
I suggest that you write to the company, saying that you are not satisfied that the repair for which you have already paid has been satisfactorily carried out. Make the point that by the company's own admission it has already fitted a keyboard incorrectly on one occasion, and say that you intend to pursue the matter vigorously. Say that you reserve your rights in law, and that unless you hear from them within 7 days from receipt of your letter, with a satisfactory proposal of how they intend to rectify the matter you will immediately institute proceedings against them for the recovery of your money, and for damages incurred as a result of being unable to use the machine. Say that you will employ the services of a third-party expert to determine that the keyboard is incorrectly fitted, and that you will add the cost of this advice to the claim that you make in the small claims court.
Send the letter by special delivery - not recorded delivery.
If you receive no response to that letter you'll be faced with a decision - either to plough on and make good on your threat to start legal action, or find someone else to fit a keyboard and stand the loss. It's a stark choice, and of course you would have to convince a small claims judge that the keyboard was indeed wrongly fitted - this may mean finding an expert who is prepared to confirm the fact.