Long story short, while turning the engine over on the starter of my old CBF there's no spark, but when you stop cranking there's a big healthy looking spark just at the point the engine stops spinning.

All the cables and connectors, switchgear, etc appear good. A known good spark plug didn't make any difference. The battery is a little tired so I tried a larger fully charged one. The voltage at the starter motor appears ok and there doesn't appear to be anything draining the battery beyond the lights when the ignitions on.

My prime suspects are the ignition switch or something to do with the CDI ... any thoughts?

I don’t know these bikes, but the symptoms resemble a faulty primary circuit on a bike fitted with a balasted ignition system.
Coils on these systems are designed to operate at a lower nominal voltage than the circuit voltage. The primary ignition circuit is divided into two parts with separate functions: in the principal circuit of the two current is routed through a resistor, known as a ballast resistor. This is wired in series with the lower voltage coil so as not to overload it. However, during engine cranking the ballast resistor is bypassed and all available battery voltage is applied to the coil which gives a better opportunity for a strong spark at the spark plugs at a time when the bike’s electrical system is under heavy load.
Your symptoms indicate that the part of the circuit that is used during cranking (unballasted) is open and hence there is no spark.
As I say I don’t know the bikes, but I do know that ballasted ignition systems have been in common use since the 1970s and the symptoms you described fit the fault I have presented.
Check your wiring diagram to see if your bike has a ballasted ignition system and then look on the motorcycle’s ignition system for an open circuit on that part of the circuit that bypasses the ballast resistor.

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You look at where you're going and where you are and it never makes sense, but then you look back at where you've been and a pattern seems to emerge. (Pirsig)

Excellent, I'll go check that thanks

The extent of my electrical prowess goes down considerably on bikes that don't have a kickstarter

EDIT: If it's the ballast bypass then I should be able to bump-start the bike ... I'll give that a try later.