MongoDB connection timeouts (Azure)

I've got an app hosted on Windows Azure with the MongoLab add-on and I have an issue where the app works perfectly well after I've deployed it, however after a period of a couple minutes inactivity I get a database error:

No replica set primary available for query with ReadPreference PRIMARY

This error does not go away until I restart the website where the same pattern returns - works perfectly until a couple idle minutes when the error returns

My basic app code:

mongoConnect(process.env.CUSTOMCONNSTR_MONGOLAB_URI, {
server: {
    auto_reconnect: true,
    socketOptions: {
        socketTimeoutMS: 120000,
        keepAlive: 1
    }
},
replSet: {
    socketOptions: {
        socketTimeoutMS: 120000,
        keepAlive: 1
    }
}
}).then(function (db) {
    //app configure ...

    http.createServer(app).listen(app.get('port'), function () {
        console.log("Express server listening on port " + app.get('port'));
    });
});

*mongoConnect is a simple "promisified" function of mongoclient.connect

I understand this is best practice, I'm establishing one connection to the db and then reusing that db object for all requests. From reading the likes of https://support.mongolab.com/entries/23009358-Handling-dropped-connections-on-Windows-Azure I've set my sockettimeouts to 2 minutes to prevent any firewalls prematurely killing sockets and auto reconnect to true so they reconnect, however neither setting solves the problem.