Wonderful. I'll try this out over the weekend - I saw your conversations and was meaning to ask you what the conclusions were! Thank you.

shs and sign are the keys I'm using for a testnet. By setting sign, the network can not be linked to the mainnet. Setting sign to null means your network could 'join' or 'leak' to the mainnet, depending on how you perceive that!

I'm very new to this so apologies for the very naive questions. I'm slowly making out shapes in the fog but I'm still easily confused :)

By mainnet I presume you mean the main SSB feed that is synced with people you follow and who follow you (or to put it in ever simpler terms, the place where the posts that you make and read live). Have I understood correctly?

By testnet I presume you mean an independent feed to this that is not synced (by default) with the main feed. Is that right?

Is it possible for two testnets (both with the sign set so they cannot join the mainnet) to sync on the same LAN (and/or via a pub running a test network) with one another? In other words, is the testnet an actual network or is it just local to the machine running it?

What does it mean for a testnet to "'join' or 'leak' to the mainnet"? Do you mean an sbot server running on the testnet (with the sign set to null) might be visible to an sbot server running on the mainnet on the same LAN (or via some kind of common bridge like a pub).

Please don't hesitate to tell me to RTFM if this is clearly documented somewhere and I've missed it!

On the topic of testing, the following instructions might save someone some time if they want to set up an install in a VM. I put a clean install of Ubuntu Mate 16.04.3 (64 bit) on VirtualBox and used the following to get the easy install of patchbay (which runs an embedded sbot with all the right plugins already installed) running.

Note 1: I haven't installed the dev set up yet but the dependencies should be more or less the same
Note 2: @mix recommended using nvm to install node instead of curl + apt-get - https://github.com/creationix/nvm

With those caveats in mind, these are the basic steps that worked for me:

sudo apt-get install git m4 libtool eclipse-cdt-autotools curl 
curl -sL https://deb.nodesource.com/setup_8.x | sudo -E bash -
sudo apt-get install nodejs 
sudo apt-get install -y build essential
git clone https://github.com/ssbc/patchbay.git
cd patchbay
npm install
npm run rebuild 
npm start