Adrian Brentnall formulated the question :
It's for a 'safety' relay on a glass-fusing kiln - will be wired in-series
with the Big Relay that switches mains to the heating elements.
In the event of the Big Relay failing 'on' (which has been known to happen),
the kiln controller (12v out) can drop out the safety relay and avoid a
melt-down. Safety relay needs to be rated to break 15 - 40A at 240v (several
different kilns with differing specifications).

I assume the concern is the Big Relay's contact becoming welded up?

Why not just have two 240v relays, both being switched on by the
control supply and both having their contacts wired in series? Even if
the contacts of one weld up, its very unlikely both would weld up.

I have a circuit which actually uses two in series for switching a high
DC current (100+ amps). One is delayed switching, so the other relay
just carries current, rather than switching it.

A separate circuit monitors the main switching relay, so that if it
does break when its coil supply goes off, it prevents the circuit which
normally powers coil, from powering the coil - an LED then comes on, to
indicate a fault. Human intervention is then required, to unstick the
relay.

Another thing you could do is use an electronic zero switching relay,
much less likely to latch up. I have had one here in constant switching
service for a couple of decades.