Binding brakes, or something else?

It sounds like a simple seized caliper to me. Just check the sliders are free to be sure.

If you replace one, replace the other to ensure even braking, as has been said.

Kirkynut

It wouldn't cause brakes to pull, but the woosh-woosh sound you can hear when rotating the wheel isn't just the back-plate touching the disc is it?

I had the exact same issue on my SAAB 93 - the back-plate was so badly corroding and the metal exfoliating so when it had been sat for some time it made a proper din! I attacked it with a screwdriver scraping of the exfoliating layers of rust and adjusted (bent) the back-plate further from the disc.

I overhauled my Jimny front brakes the other day thinking I'd got warped discs. The run-out on the discs was minimal to non-existent but I found the slide-pins although in good shape were very dry with old anti-seize grease. I replaced them regardless and used the sachet of grease in the kit and my brakes have been transformed.
Exercising the pistons in the calipers could help, but failing that there is a caliper refurb kit available.

The whoosh, whoosh noise is the pads continuously rubbing on the disc from the seized caliper not retracting.

Kirkynut

I know its the front left, as that is where the smoke comes from

Could not push the piston back easily. Looked like my tool was about to break, and I moved it 1mm. So I ordered a new caliper.
Already tried replacing the slide bolts before posting, so I know they are good.

You could try pushing it out a little if still connected to hydraulics and the pads off and see if that helps it to move back more easily.

Some times the flexi hoses collapse internally and act like a one way valve allowing fluid to pass to push the piston out but not allowing it to return causing the piston to lock.

If you can push the piston back with a bleed nipple open then this is where your fault lies.
A new flexi hose is cheaper than a calliper so it’s worth investigating.