The Health and Safety at Work Act 1974

states in section 2(1)

"It shall be the duty of every employer to ensure, so far as is reasonably practicable, the health, safety and welfare of all his employees."

Two parts of that sentence need explaining. The first "It shall be" is the easiest. In legal terms "it shall be" = you must. By law you must carry out your obligations to your employees.

The second part "so far as is reasonably practicable" is a little harder to understand and is best explained by an example.

An employee falls off a ladder whilst pointing around an extractor fan fitted to a gable end and breaks both legs. The H&S inspector turns up to make his report. Did you try to ensure so far as is reasonably practicable the poor lads health and safety?

Perhaps you might have had someone footing the bottom of the ladder. Its a step in the right direction but still isn't really enough. You could have had someone footing the ladder and issued the lad a safety harness that can be clipped to a rung. 

Why didn't you use a scaffold tower? Perhaps their was only an hours work and the cost of scaffold couldn't be justified. It is up to you to weigh up the risks involved against the costs and try to find a balance that is reasonably practicable.

For a job that might take minutes rather than hours, using a ladder is obviously the most cost effective (until you get hit with an industrial injuries claim) but before making that decision you need to analyse the risks involved in using the ladder so that, after taking the precautions to minimise those risks, you can balance the risk against the cost of the alternative, i.e. scaffold or cherry picker.