Hello fellow members,
I signed up here after reading about quite a few people having to get replacement Nexus 5 phones for various reasons. I bought my phone through Google Play Store on-line and that's where I made my complaint about loose power button and poor sound, After 2 email exchanges with with Tech support I was passed over to Google UK in Ireland...I have put the jist of the email and instructions below so that anyone who has same problems will have a better idea of the process. I did make sure I had enough money in my account and was glad I did because the it seems the cost of the phone has "disappeared" from "available balance". On the Google Play account page it says 3 to 4 weeks for delivery and if it is that length of time plus an few days on top to get the old phone back...That is a long time to have £350 on hold.
Hope I have helped some of you out.

GOOGLE:
I'm sorry to hear that you need a replacement for your device.
Please follow the instructions below to order your replacement and return your device:
Step 1: Order Your Replacement Device [Time-Sensitive]
Please click through the link below within 24 hours of receiving this email to order your replacement device:

(*****************Link Removed************)

About ordering your replacement:
The replacement link above expires in 24 hours.
You will not be charged for the replacement, but your card will be authorised up to the full price of the device.
Please ensure that youíre signed in with the same email account you used to originally purchase your device.
You will need to order the replacement from your computer (it wonít work from a mobile phone or tablet).
Step 2: Pack it

Print out the attached RMA form.
You'll receive a return shipping label and shipping package in the mail within 5 days.
Make sure that you keep the SIM card for your device, if applicable. Any SIM cards that are returned with the device will be destroyed.
Please do not try to ship your device back to us before receiving your replacement; wait for the prepaid shipping materials.
When you receive the prepaid shipping materials, put all of the original itemís packaging and accessories along with the printed RMA form into the shipping package.

Step 3: Ship it
Drop your package off at your local post office.