First question….”What is a Sonos 2?’

As a guess, they mean S2 of the controller. The questions become how good is the WiFi reception 80 meters away, and what is the power situation. If it isn’t good, something would need to be done to strengthen it, either via a ‘mesh’ device perhaps midway, or running a Ethernet cable to the location. Which also then defines the speaker. Sonos spdoesnt make a true ‘outside’ speaker, although I think they do sell versions from Sonance, which require wires from a Sonos Amp. 

The two battery powered Sonos are the Roam, and the Move. Both of which can be used, after their set up on a home network, as a non-WiFi connected Bluetooth speaker, if that’s the way you wanted to go. Neither are designed to be left outside, however, but can be used there. 

Run Ethernet and add an Access Point in the garden?

As my Sky router is in the middle if the house, there is a Sonos port near the rear of the house. Could I use an Ethernet socket on the port to run an Ethernet cable down the garden house and connect an Ethernet switch? Would this give me a a solution?

That would still require the Port to be connected to your router via Ethernet to create the SonosNet which is a wireless proprietary 2.4Ghz band. The question would still remain is whether or not the SonosNet could reach the garden house. Additionally, Sonos new stand-a-lone speakers such as the Era 100 and Era 300 don’t recognize the SonosNet. 

Therefore, your best option is to run Ethernet from your router to the garden house and connect it to an Accesspoint to extend your Wi-Fi as suggested by ​@Stanley_4 

​@Redlazza - Bruce ​(@Airgetlam) asked earlier, what is the power situation in the garden house?

There’s no point running ethernet to the location if there isn’t power sockets available.

I would hope there is some type of electrical current to the garden house even if it’s just a ceiling light bulb fixture with a pull chain. Electrical for one or two devices could be supplied with a product as shown.

If no electrical at all….it's just a room hopefully with windows to allow sunlight during the day and lit by candles at night. 

EDIT: I assume there is a similar product in the UK 

Power at the garden end isn't a requirement as many Access Points accept POE (power over Ethernet) and the standard maximum distance is 100 Meters. That can be extended too.

My AP is over 50 feet from where the power is addd to the Ethernet cable it uses. I have also used POE for remote cameras, nice to just pull one cable and have power and data.

That solves the problem for electronics…. but what about light . Yeah...I know PoE lighting like this  

Edit: We’d better stop this conversation as it’s borderline hijacking...I think