2006 enterprise glow plug relay

Hello everyone, 
Apologies of this is a worn out subject!

I’ve just bought a 2006 berlingo enterprise 1.9
When starting I notice there isn’t a glow plug light. 

Could any of you clear up wether it’s factual that the light only comes on below a certain temp? From what I’ve read this is contradicting. 

The van seems to start fine apart from two cold mornings in the two weeks I’ve had the van where it was a little longer than you’d perhaps like. 

I have 3 months warranty with the van, wondering if I should pop back and ask them to replace the relay or not?? 
Or is it easy to do myself, I’ve seen the part for approx £30. 

Thanks for any help 

Ben