for me i found seeing them before the test does to he an issue so i found every practice clip online and i practiced till i worked out what works best for me.
end of the day the hp is designed for brand new drivers of cars. it is same test just longer for hgv so those of us who have been driving notice alot sooner.
for me i found clicking the whole time i saw the hazard worked best but you have to time the clicks so it does not fail it from being clicked to many times to quickly. so my advice would be use practice clips and try work out what method works for you.
if it was timed to when you would react in a hgv it would be easier as then it would mean those of us who see them sooner would have a better chance with less clicks as it would expect a click sooner rather than the timing it has