Your GI if you're running a 3 speed is still high with a 45t chainring, having said that, SJS have a 50t one.
https://www.sjscycles.co.uk/chainrings/34-thorn-110mm-bcd-5-arm-reversible-single-chainring-332-inch-black/
Running a 2-speed, Ed - That changes things slightly - does it not?
Slightly but not that much, if you find yourself spinning out too much with 54/12, then stick with 54t.
@JurekB started
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Your GI if you're running a 3 speed is still high with a 45t chainring, having said that, SJS have a 50t one.
https://www.sjscycles.co.uk/chainrings/34-thorn-110mm-bcd-5-arm-reversible-single-chainring-332-inch-black/