I was wondering if anyone was having problems with signal strength and selecting GSM only for the network?

I'm on Vodafone and when the phone is on 3G and GSM, the signal strength drops out when entering any buildings (I live in Nottingham). So I read somewhere on the net that the cause could be the inferior 3G signal strength and this can be overcome by selecting GSM only.

When I select GSM only, I can't pick up any network. When I do a search it finds all networks except Vodafone. Does anyone know why this is or if I need to do anything else to make it only use GSM?

Thanks