We will have some maintenance work to do in water storage chambers and I want to decide whether this can be done by our in house team (tradesmen) or a contractor. This will become a regular occurrence – 3 or 4 times a year. The chambers are constructed from preformed concrete tubes and a concrete lid fitted with a manhole, they’re about 2 m deep. Obviously, our main concern is the confined space. The chamber holds spring water for a private water supply before it passes through a treatment system, but will obviously be drained for the work to take place. I’ve taken a look at the Water UK guidance note on the Classification and Management of Confined Space Entries and L101. I’d welcome your thoughts on air quality/sampling; PTW; and rescue plans and equipment. In particular, what are the real risks of oxygen deficiency and noxious gases? I’d like your opinions on how best to proceed before spending a lot of time and money on kit and training. I appreciate that it might be difficult to give a definitive answer given the scant information.

Portable oxygen deficiency alarms are inexpensive.

You don't say what the maintenance work is that you are going to do in the space. Obviously you need to consider this as part of your risk assessment in case the work you are doing is introducing additional risks to those in the space itself.