To quote the HSE indg455 "Safe use of Ladders and Stepladders": "The law says that ladders can be used for work at height when a risk assessment has shown that using equipment offering a higher level of fall protection is not justified because of the low risk and short duration of use; or there are existing workplace features which cannot be altered. Short duration is not the deciding factor in establishing whether use of a ladder is acceptable or not – you should have first considered the risk. As a guide, if your task would require staying up a leaning ladder or stepladder for more than 30 minutes at a time, it is recommended that you consider alternative equipment. " INDG368 Using contractors clearly states that it is the responsibility of both companies. "Both you and the contractor you use have responsibilities under health and safety law. Everyone needs to take the right precautions to reduce the risks of workplace dangers to employees and the public. Make sure everyone understands the part they need to play in ensuring health and safety." Hence the need to pre-qualify any contractor, if they are not offering a suitable safe system of work then they should not be engaged. I would be in full support of your recommendation for the use of a suitable MEWP in preference to a ladder, as you state its a long way to fall. Even being simplistic about all this, I doubt they will decorate it within 30mins - ok 30 seconds for each decoration - but how many decorations, I personally would count the accumulative time. I would suggest that both parties would be held equally liable, especially as the hazard has been recognised and allowed to proceed as you would have inspected their procedure and safety controls. The insurers will go for anybody and everybody! Good luck.

Was a bit quick with the send button - I'm not really illiterate ;-)

Here we go again - Christmas is a time of year when people seem to leave their common sense at home and act carelessly. The task is working at height. There will be a WAH risk assessment That assessment demands looking at safer methods/equipment A 7 metre ladder is not as safe as other equipment. Do the right thing and arrange either something or someone else for the work at height. No worries about liability if you get it right first time.