We don’t need to see debt collector letters. As you have been advised, they are powerless to do anything. Ignore and please don’t waste our time by showing them to us.
They have already added the fake £70 fee to the original £100 charge. If this ever went to court, that fake added £70 would not be allowed.
If it ever got to court and a judge decided that you owed a debt to Gemini, it would be in the small claims track of the county court where costs are fixed. Of course, if the judge decided that your defence was valid, you would not owe them a penny and you could also ask for your own costs of up to £95 and any mileage/public transport/parking costs.
The fixed costs are the claimants £35 court fee and fixed legal costs of £50. So, in the worst case scenario, the amount at risk is ~£185 which is less than the claim would be for. Also, as long as it was paid in full within 30 days of judgment, there is no record of this on your credit file. It is completely expunged. The only way someone gets a CCJ on their credit record is if they fail to pay it within 30 days.
So, don’t show us any more debt collector letters. Do come back if you get a Letter of Claim (LoC) which is different from a debt collector letter as it must give you a minimum of 30 days to pay. A debt collector cannot send you an LoC. It must come from the claimant themselves or their appointed solicitor.