All out internal fire doors are propped open during the day (it's a small office block) on an evening the person who locks the doors does a final sweep and ensures all wedges have been removed and the doors are closed.
It is written into our safety induction procedure and our FRA.

I find it very hard to believe that a prosecution could arise having this process in place.

My experience backs up what hammer1 is saying. At one facility I worked at (not a construction site) they were getting 4/5 of these letters at a time. Fire doors being wedged open were one of them and after the initial panic we realised they are more of a statement of the issues rather than a threat of prosecution.

To cut a long story short, the ex-responsible person was very poor and a lot of work was done to bring the fire strategy up to an acceptable standard in a short space of time. The “door wedge” is a never ending battle however through a mixture of inspection, bulletins, toolbox talks, emails to offenders and the involvement of line manager incidences were dramatically reduced. The company policy was also rewritten to shift some of the burden off the HSE manager to senior line managers, all with the backing of the Managing Director.

With regards the legality of wedging fire doors we reminded people of article 23 of the RRFSO and HSW Act reg 7 & 8, and showed them this http://www.manchestereve...-after-care-home-857938.