There is some additional information in the ACoP at http://www.hse.gov.uk/pubns/priced/l101.pdf but I think you have answered your own question when you referred to assessment. We do vertical entries into chambers on sewage works and we require 2 safety men at the top, as in the event of an emergency there is a signifcant workload to deal with such as recovering the worker, calling for emergency assistance, giving first aid etc. Other types of confined spaces work may not need 2 safety men.

It will also depend on issues such as whether the person in the chamber is attached to a tripod and can be raised directly upwards.

Lots of guidance at

http://www.water.org.uk/...pdate-ed-2-2-oct2009.pdf