gas fire selection

We moved in to our det. 1971 built house just over a year ago. It has an outset gas fire placed on a rough brick/stome fire surround which from the shape of the pot on the roof (aladdin style top) seems to be a prefabricated flue. We want to replace the fire as we believe it is rather elderly and being outset it intrudes somewhat in to the room.
Enquiries made lead to various opinions about our options. Some say there will be little or no problem whilst other experts draw in their teeth with the builder's whistle and say that because of the nature of the fire surround with irragular surface a proper seal won't be possible. They then suggest a catalytic gas fire or an elecric (with higher heating costs but lower (?) initial outlay. Any comments and advice would be appreiciated please.
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