Radiator size calculators

My house has a kitchen that was extended and doubled in size by the previous occupants. Unfortunately they did not change the central heating radiator at the same time and consequently it is not up to the job. The radiator that is presently in situ is a double (without convection finning) and is 750mm high x 1440 mm in length. I need to replace this radiator with a more efficient one and in order to determine the required size and type I have made use of three on-line radiator calculators. I have entered the same information into each of these in relation to room size (4.5m x 4.5m x 2.4 height) and number of outside walls (2) and size of window area (2.8 sq m). Problem is they all give different heat requirements varying from 5175 BTU/hr to 10060 BTU/hr !!
Two were fairly close with one giving a specific 5998 BTU/hr and the other giving a spread of 5175 - 6200 BTU/hr.
Any heating engineers like to give me their view of the above and a recommended replacement radiator. (The Stelrad compact 450 x 1400 is a similar length to my existing radiator but much more compact in height and yet gives out 6549 BTUs which would appear to be more than adequate for my requirements.)

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