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Any heater in this position is going to be constantly fighting against the cooling effect of the window, particularly immediately around the existing extractor. It’s a good idea but I doubt what you have in mind will work in practice.
Can you better seal the shower cubicle in any other way? Or fit an additional extractor in the cubicle, perhaps boxing the ducting in?
Sorry to say it but the best chance you have here is to replace the window with a smaller one and fit a suspended ceiling with a ceiling extractor or two.
I am struggling to find the right product, though. The closest I've got is a "skirting heater" but I've not found one that is small enough and rated for use in a bathroom.
I'm wondering if a wall-mounted vertical radiator would be better.
The idea of it being electric is to avoid over-using my central heating when I only want to heat one room.