Hi. If the speakers are stereo paired then the volume controls do not act independently. You cannot change the volume of just one speaker so I don't understand the question. 

I tap the volume increase on the SL and it I creases it's volume. Maybe I've set them up wrong. Took ages to get the Sonos One working from the echo dot. Sometimes I do regret buying them, should have stayed with a hi fi system. I'll start them again from scratch and see how I get on. Thanks for advice

After installing your first speaker, did you “add stereo pair” or did you create a new room which you’ve then grouped with the other room/speaker?

I would try leaving the Echo Dot out if the equation initially.

Why are you using a Dot when the One has Alexa built in? (There are possible good reasons, but I am trying to understand what you want to achieve.)

I set them up as a pair, only thing I never bothered with was the true play. I’m going to do them from scratch again tomorrow.

The echo dot is far better than the mic on the Sonos, it suits us better as it’s placed next to were we sit and easy to give commands to. Speakers are on floor stands 3 mt away and about 3 mt apart. We found we had to really shout commands quite loudly. So added the echo dot for commands, mic is switched off on Sonos One.

Those are some of the good reasons I referred to!  But I think it would be worth trying briefly without any voice control.  I would certainly create the stereo pair in Sonos before discovering devices.  

This morning I reset both the Sonos speakers and paired them up in stereo without the echo dot, they worked fine, using the Amazon Music Unlimited App, and also with Alexa using the Sonos Mic. Unfortunately the Sonos mic is still poor at hearing commands and after a bit of a struggle I have the Ech Dot connected to control them again, all seems ok.  

It is never a good thing to reset your speaker, unless you are transferring it to another party.  It does nothing a reboot would not fix, and it erases valuable diagnostic information in case something really is wrong. 

It’s done now, seems to be ok at present. Going to buy another SL to replace the One. Will use that in another room on its own. I love the sound of Sonos, but find them very fussy when used with streaming radio, I really miss my old hi fi with a proper tuner, CD player,turntable. Life was much easier. I regret giving in to family pressure to go down the streaming route. I can’t say I’m a great lover of Alexa either, but I’m outvoted on that.