A few issues to work on..

[David]

I've done the 'demist' test and the radiator fan does not spin up. It does come on when it's hot though, but I'm guessing one of the speeds is broken?

Probably the fan resistor needs replacing.
https://www.dmgrs.co.uk/products/rov...airs-pgj000110
Bumper-off job, but easy enough to do as a DIY task.
http://www.spunagain.co.uk/ZT260fan.html

Coolant leak from the manifold and the last few days I've been having rough running issues.

Probably coolant in the plug well recess(es). Detach a coil from some of the plugs in the area of the leak and see (single special locating bolt, 7mm I think; careful not to drop it). No need to detach the electrical connector on the COP to be able to pull it off the sparkplug and out enough to look. Probably soaking wet. If you have a OBD-II tester, you will probably see P-codes pending, showing precisely which plug(s) are misfiring.
If so, look towards fixing/replacing the leaking manifold ASAP, otherwise the long springs in the coil-tubes which connect to the plug-caps, will start to rust, necessitating replacement of the COP. Anyway a leaking manifold will only get worse, whether it be a gasket or "plastic rot"

David