Best cheaper sat nav for a delivery driver

[LukeDM]

I have just started a temp job as a grocery delivery driver and I need a sat nav but I don't want to spend much (if any) more than £100, the job is supposed to last 12 weeks. I would use my phone but I work in north Devon so reception is very poor as most delivery are to farms and tiny villages. The nav needs to be good at finding the fasted roots including small roads and farms in the middle of no where. Being able to use grid references would be a massive advantage! I really have no idea about sat navs so any tips would be appreciated, thanks.

[LukeDM]

Cheers chaps, I'll need to get my phone fixed before I try any of those but I'm pretty set on a stand alone device, I'll ask some of the guys at work what they use.