I realise that lots of users will be very reluctant to swap out the Plusnet router - such an action moving the install away from "standard" with potential for problems if service issues arise in the future.
I have recently come across a device that will provide a guest WiFi network with a good level of security.
The device I have been "playing" with over the past week is a Devolo dlan 500 wireless access point.
The device can be quite easily configured with a Guest Network Access using the excellent Devolo Cockpit application.
A dedicated SSID and password can be specified and other functions are configurable such as "Opening Hours" or "Max length of Logon for a connection" and "Only specified MAC addresses"
An IP sweep from a logged on guest gives only two visible addresses, other than any guest connections.
These are the IP address of the device itself, which seems to be impervious to any further probing and the IP of the router which brings up the login page.
So, if you keep the router login details safe (you have taped over the information on the router itself haven't you?) then you will keep the details of your other connected devices away from prying eyes.
There is a comprehensive manual available for download from the Devolo website
http://www.devolo.com/products/dLAN-WLAN/dLAN-500-WiFi/data/Manual-dLAN-500-WiFi-com.pdfReading the manual it states quite categorically that the guest network devices can only access the internet. If, however, access is required to the private network then there is an option to permit it.
Incidentally the manual is 80+ pages and goes into detail regarding the configuration options
Most impressive compared to the general low grade literature usually provided these days.
Devolo are at the pricier end of the market, but used ones do appear regularly on fleabay.
A few days ago the wireless device sold for £28 and non-wireless devices start a few pounds less, but can be more expensive if they have mains pass-through socket with multiple lan ports.
One thing to note is that the guest network option only applies to the 500 and 1200 range of WiFi devices.
Simplest install is where you have a spare router port and a nearby mains outlet.
If you lack either, or both of these, then Devolo sell a number of other devices with mains pass-through sockets and/or single or multiple ethernet ports.
If you need this extra device then it gives you the added advantage of being free to re-locate the wireless device to a more optimal location for your home as it can have your private WiFi access working in parallel with the guest access.
Disclaimers :_
I have no connection or affiliation to Devolo other than a satisfied customer
Although I have some some network knowledge I am not a network security expert
HTH