I would go for above the fireplace

I think you should use the new mirror above the fireplace. The wall in the kitchen would suit a wider oblong mirror hung landscape. Will you be having a dining table under it? You would want the mirror ideally 2/3 the size of the dining table so it doesn't look 'lost'.

Thanks everyone for your recommendations/advice. I definitely want a mirror facing the kitchen so you can see the kitchen from both sides as you enter the space. I'm not having a dining table facing the kitchen as I have a dining area just before you enter the new kitchen extension. I was planning having some sort of seating area though and will have bar stools at the breakfast bar.

I just can't seem to find what I want and have been struggling finding a suitable mirror to face the kitchen. This Next silver pewter mirror I've bought is 1.2m by 0.8m and I was planning to hang it landscape but you have all recommended hanging it above the fireplace ;-). I wonder if you have any ideas of what sort of mirror (colour, style etc) would therefore look better on the wall facing the kitchen.

Thanks again all - much appreciated.

Maybe take the mirror you have already above your fireplace and paint the frame the same colour as your kitchen cabinets and hang that in the kitchen?

The silver frame on the new mirror compliments the colour of your fireplace better than the existing gold.