My old - but still looking good - gas hob is a Thorn.

The spark generator looks as though it is on borrowed time as the housing 
has melted badly at some time (I blame the chip pan) due to heat being 
reflected downwards through the hob top. The makers info is "TURNRIGHT 
GL830" It measures 3.5 x 2 x 1 and has 4 HT terminals.

Some generic ones may not fit the space.

Does anyone know of a good source of such spares - used / cheap. The hob is 
1988 vintage. It will get replaced when we get around to doing up the 
kitchen.