Quite suddenly the waterflow from my hot water tap became restricted - there is now very little water flow. I am not sure if it happened instantly or over the course of a few days. I am assuming some sort of debris or scale is clogging it up. Where would this end up? Near the tip or inside the steam boiler?

The filter unscrews, so it isn't hard to eliminate that as the source of the problem.

On my Vivaldi the solenoid valve for the hot water develops some scale that can restrict flow. If your problem is not the aerator at the end I would open the solenoid valve and clean it with some white vinegar.

Your machine may have a different "issue" than my Vivalid...

I unscrewed the filter but nothing there. No sign of scale buildup either. What's the next logical place to look?

I would try drawing some hot water with the filter removed, just to be sure. If there is indeed scale blocking the pipe, it's impossible to say where it might be internally. I would also be concerned about scale in other parts of the machine if it's the bad.

Descaling the steam boiler might help dislodge any build-up in the pipe. You could also try blowing air through it cold, I guess?

Try taking the pipe from the steam boiler off the water valve. Is the valve plugged? Can you blow through it? With what? If the valve is plugged, take apart. You can find info with google. The valve, either the original or the newer one is relatively easy to take apart. Then the steam tank can be inspected when cool by removing the water level sensor (wire 90deg bend in the top of the tank). What does it look like inside? If the lines are plugged, it could be calcification depending on the water used for the machine. For cleaning the steam tank, http://coffeetime.wikidot.com/izzo-alex ... -descaling should get you started. In the uk is a great resource.
Cheers