Etrex 20 is great for navigation in my opinion. Exactly what you need. It’s a steal if you can get it for £85 too. Last time I looked cheapest was Amazon for £130. Most places sell it for about £150. But you have the new ranges of Edge units out now, so perhaps they’ve dropped a bit.
Got myself an Edge 500 in the black Friday sales recently, and what a pain in the arse it is to use. I know it’s not meant to have proper mapping functionality, but the breadcrumb trail keeps disappearing. I found it very difficult to navigate a route I mostly knew already (obviously there were bits I was unsure about). That and the initial setup is a bit of a learning curve, with things like average speed missing from the default settings.
Etrex works pretty well out of the box. I have mine setup with a few custom settings, but all the basic stuff you need is there. Only fiddly bit is the joystick when you’re on the move.
The Garmin mount works fine for me. Clips in solidy. But…I have lost one! Don;t know how. It was a very cold day. Had it at cafe stop. Definitely walk out with it. May or may not have left it on the wall outside. May or may not have clipped it in properly (which to be fair would be easy to do – especially when your fingers are numb). So I join the ranks of lanyard user with Etrex numero 2 (which thankfully I got for a good price in the black Friday sales last year).