Pictures are needed.

Here is the result of a Houzz search on covered radiators. Maybe one of these photos will inspire you. Hard to respond without pictures of the actual elements.

https://www.houzz.com/photos/query/covered-radiators/nqrwns

I am not sure I would use long curtains with this window. A nice valance and shades might work best given the radiator. There are lots of photos on the link I posted that would work to disguise this.

I don't think I'd use anything except a minimal blind for light control -- great view, and you'll just mess it up with more (not to mention the fire hazard of long drapes across the radiator)...

You have a great view! I wouldn't consider drapery in such a small space. If all you are looking for is to 'sleep-in' and you need blackout, I would suggest a blackout honeycomb shade. You can mount them within the opening and have a valance made that would cover the shade (out of sight - out of mind)

There are some very nice fabrics out there for you to create a french-country look and still keep the space clean - bright and airy.

Good luck!

Thankyou everyone for your comments and suggestions. I have the long curtains and I don't want to change them. Still might consider a blind and keep the curtains open... lots of food for thought. Agree about the view! But it's one we can see from every window in our house so I'm not fussed on shutting it out for once! Shame it doesn't pay the bills ;)