Marsh Farm Essex / Great Days Out

Today we went to Marsh Farm in Essex and presented our voucher to get a child in free.  This was declined as the voucher does show some large venues codes but not the name of Marsh Farm. They are generic vouchers but the venue did not seem aware of them and when called later were unaware of the CC vouchers.  Has anyone a similar experience please.  This is my first attempt at using a great days out voucher so not so successful.