Sonos One and SL alexa issue

We have a Sonos One and a Sonos SL paired in stereo, these are controlled with commands  by an Amazon Echo Dot. This set up has been working perfectly for a long time now, but the other day it started to play up, on asking Alexa to play for example Radio 2, it would only play it on the Echo dot and not the Sonos. Nothing had been changed in the set up, we unplugged the Sonos speakers and echo dot rebooting them a few times, that didn’t work. I didn’t set the system up, we got someone in to do it, but hes no longer available. We switched them on this morning and they are sort of working again, but before when we gave the echo dot an instruction the reply always came via the Sonos speakers, it now comes from the eco dot and not the speakers, ie... Alexa play radio 4, echo dot replys playing BBC  Radio 4, and the station comes through on Sonos speakers about 6ish seconds later. Any idea why this has changed and how to get it back to previous set up. I really like the Sonos Sound but not the faffing about setting them up. Any advice appreciated