i want to replace a gas fire. Will a gas engineer be able to buy the model i like or do I have to go to a gas fire shop.

You can buy the fire yourself ,or the gas engineer, but you will need him any way to fit it. Needs to be Gas Safe registered engineer.

I would recommend calling out a Gas engineer first to look at your existing Gas fire to choose the most suitable new gas fire in accordance with your room size and dimensions of the fire. Once theyve had a look they can advise you what to buy or can buy it for you and fit it aswell . Always use a gas safe registered engineer. From Neymar

You would need to have a small survey visit where the engineer takes out an existing fire, measures the catchment area dimensions, looks up the chimney, smoke tests the chimney and considers ventilation if the fire is to be over 7KW. Once you know all this info, the engineer can look at the installation instructions online for any fire that you like, look at the dimensions required and tell you if your dimensions are suitable. If not suitable, building works would be required to alter the dimensions or just find a fire that can be fitted in your existing catchment area/opening. Once the engineer gives the go ahead, you could order the fire or they could but don't order a fire only to find it's totally unsuitable and doesn't fit. Do the research first or you'll be returning a gas fire in the post!

he will need to be gas safe engineer and hold the relevent gas element on his card

any gas engineer should alert you to the fact that you need to buy a gas fire that fits your requirements and the type of chimney you have.when looking at the fitting of gas fires if a customer requires a certain fire i advise that before buying you check that it will replace or fit what you already have.