I work for Pilkington so wanted to clarify some of your points.
If the contract was for Pilkington K Glass and anything else was fitted without your prior knowledge, this is called "passing off" and trading standards should be able to advise you on your course of action. Pilkington K Glass is the best selling and most widely known Low-e glass in the UK&Eire market (a Low-e or "Low-emmissivity" coating means it is an energy saving glass providing thermal insulation to keep your home/conservatory warmer in winter). Other people try to "pass off" their products as Pilkington K Glass as they do not have a well known brand.
Guardian is a glass manufacturer (Like Pilkington) and produce a range of glass products. They produce ordinary clear glass which does not have any thermal insulation properties. They also produce a "Low-e" glass which does have thermal insulation properties.
If you have ordinary clear glass then your conservatory will definitely not perform as you specified it should.
If you have another low-e glass then the issue is more about you not getting the glass you had specified.