Thanks for the reply, I think I'll wait for the second beta, strange though as it has been publicised that watch os5 is S1 and above.Interestingly Apple's developer site says "watchOS 5 beta supports Apple Watch Series 2 or later", which suggests Series 1 isn't supported in the beta yet. This could just be a typo, but the release notes indicate some things that don't work specifically on Series 1, so maybe they haven't got support enabled for it in beta 1.
Having said that, my iPhone is on iOS 12, the Walkie Talkie app shows up on my S1 which is still on watch os4 beta5!