Can't really tell much from the video. However, I've had similar in the past but it turned out to be the pads moving in the calipers. Pull the brakes on really hard and repeat the bounce test. Also, try testing by bracing the front wheel against a wall or similar to avoid having to use the front brake at all.
You're a genius Mike.... it is clearly related to the brakes not the forks. Doing the bounce test up against a wall there is no clonking noise. Doing it with the brakes on (no matter how hard) and the noise returns. I guess it is most likely the pads moving in the calipers - which is hopefully nothing to worry about.
I've recently had the wheels powder coated with new bearings, new Brembo disks and new OEM pads. The brembo discs have more buttons than the standard discs, so I guess there could be some movement in the discs, although I think the pads are more likely. Difficult to see for sure as a one-man job. The headstock bearings seem to be fine, with no tight spots or other discernable problems.
Ive decided to overhaul the forks anyway - its a while snce the fork oil was changed and the seals and bushes have never been replaced as far as I know. I've ordered OEM parts and a set of Hyperpro progressive springs as a relatively cost-effective imrovement option - based on what I've been reading here and other forums (but mainly based on Mike's contributions). Hopefully the parts will arrive in the next couple of days and I will tackle the job at the weekend.
I'm tempted to change the headstock bearings while the front end is stripped down, but not sure there is much point if they are ok.