After serious thinking Dave wrote :
Regarding the fire risk and polystyrene - we have no kids, we don't smoke and
the house was recently rewired so I think we could be classed as a low(er)
risk household. However, just in case a fire should break out, we have mains
operated (battery backup) interlinked smoke alarms in almost every room (and
an emergency light in the hallway and on the landing wired into that circuit
that provide indication of mains failure), so I reckon the risk of
polystyrene coving is an acceptable one.

It is certain to be the flame retardant type, which only chars and does
not itself support combustion, but it does tend to give off fumes. Test
a small piece to be sure.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk