Switch bulbs

They can be replaced but is much harder to do than the later design. I would unsolder one and take it to somewhere like Maplin to help identify it. Or if your feeling technical you could convert to LED.

LED Conversion 

Thank you very much for the replies. I would quite happily have a 'bash' at converting to LED as I have taken another look at the bulb fixture in the switch, a little soldering by the looks. I'll let you know how it go's. Thanks again. Richard