Changing terms of agreement with new terms after purchase.

As per previous reviews, I experienced the same issue of purchasing a voucher from Tamalin during lockdown when they sold it at a discount and sent e mail confirmation that I can use it to pay for any booking(even discounted bookings, have this on e mail). Now when it is time to deliver they refuse to honour this agreement. It has been a nightmare dealing with them. They offered a 35% discount on the voucher, they currently have a 50% discount on cash bookings, why would i pay years in advance to receive only 35% discount when I can pay now and receive 50%? Even with proof of terms and conditions they provided prior to me purchasing the voucher they are trying to change terms now.