I always use James Reason’s Swiss Cheese model. It might not be “right” but it does explain a number of  things including the fact most of the time we get away with it  through luck; the holes in the cheese don’t align at that particular moment.

In my place I have identified 4 slices:

• Safety operating procedures supported by a process of risk assessment
• Competence of staff who can spot the gaps in the above
• Effective  management that actually supports H&S – in particular listens to competent staff
• An overarching  positive H&S  culture from top to bottom that engages with everybody and makes sure that all of the above is happening

I suspect from the responses so far from well-informed colleagues that I am not alone in not having come across the term "HSE lines of defence".  What's the context of this?

The closest I have heard of is "layers of protection" which is essentially the Swiss cheese idea and just like that, can have as many layers of protection or sections of cheese as you want or need to describe, and what the layers of protection or sections of cheese actually consist of can be adapted to the situation rather than being generic.

Now I am getting hungry.

Glad its not just me that was not familure with the term used in that way. I two use the swiss chease model - not just for accidents - tend to use the elmiments in HSG 64 as guidence to the layers, particulay those in te do section so comunucation, training, compitance etc.