As you have internet access your best bet is to download user instructions from Main Gas Appliances. If however you have lost or were not left with the user instructions @ the time of installation I would contact the installerto ask for a reason for not leaving the documents as the manufacturer always insist on this. If in any doubt visit CORGI-GAS
davegas
February 2009
Usually the top hood just lifts up and then pull forward.You should then see the two arms that hold the glass in.Slide them to oneside and the glass should come out.
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Try fixfault.com to fix home heating systems.