how can i fix my smeg a2-5 dual range cooker?

when turning the main Auxiliary knob it works fine, fan works, oven light works. Then when you turn on the main oven switch it clicks on, but lights does not light up and oven does not heat up, however if i was to stand there and hold the main oven switch in it works perfectly fine! obviously the cheaper repair would be to stand all day holding the knob in. Please can you advise what the problem is. As it seems to me that its a connection issue. Although there doesnt seem to be a connector switch on the detailed diagram i have of the cooker from smeg. its model number a2-5 s/n 50421 april 2005.....please please can you help save the exspense of a call out if its just an adjustment needed and a new part. regards sam