What type of switch is this push switch [photos]?
The diagram on the side is pretty clear -- the top terminals are for a bulb, the "middle" (one up from the bottom) terminals are a NC contact, and the bottom terminals are a NO contact.  It's almost, but not quite, a SPDT switch -- it's more like a DPDT switch that's missing some terminals.

BUT looking at the picture, it's running current from the contacts through the bulb.  It may be that the burnt-out bulb is an essential part of the circuit (possibly for safety reasons, to never energize a circuit unless there's an indicator light).

Dig through the rs-online site; this should be pretty obvious.

If all you need is a replacement, and looks don't matter, then by the diagram and the wiring, only the NC section (and possibly the bulb) is being used -- so you just need a normally-closed (or closed-when-out) SPST push-push switch with a bulb.