Activity Superstore review: Voucher for afternoon tea at the Sunborn Yacht

My sons very kindly bought me a voucher from Activity Superstore for afternoon tea on the Sunborn Yacht. In order to book the experience, you need to make a request to book a date via Activity Superstore. Each time I have tried this, they have responded by email to say they have contacted the experience provider to see if this date is available, only to respond several weeks later to say it is not available. It seems like an impossible way to make a booking. In the end, I contacted the experience provider directly to make a booking, which they said was fine from their end, but I would need Activity Superstore to authorise the booking. I contacted Activity Superstore to explain I had made the booking and ask them to authorise it and they said they would need to check that slot was available with the experience provider (even though I have a booking with them) and I was not to attend the booking unless I received confirmation of this from activity superstore. The booking is on Saturday and I have received no further messages from activity superstore. It just seems they are unwilling to let me use my voucher and it was a total waste of my son’s money.