where can I get a fire control knob for an old Baxi Bermuda 551?

I have an old Baxi Bermuda back boiler with gas fire on the front (within a wooden surround) GC No. 44-077-48. The knob to operate the fire is mounted on the top right in the wooden surround and measures about 4" in diameter. I would like to obtain a replacement knob for the fire. Does anyone have one please? Or does anyone know where I can obtain one? The knob is white with black markings to indicate how many radiants are burning.