I don't understand why it shouldn't be compatible though when the Apple site says: "AirPort Extreme works with... ...and virtually all 802.11a/b/g/n Wi-Fi wireless devices — all at the same time."
I can understand where you were "mis-led" on that statement. While it is true that the AirPorts with "virtually all" wireless devices for basic communications, it does NOT work with ALL devices for the more advanced features like extending a wireless network. This is not unique with Apple products. You will find the same for just about any vendors' products. For advanced features they are only designed to work with the like vendor's products.
What I'm trying to do is boost the signal from the Virgin router which is on the ground floor of our three storey house. So, if I bought an Airport Express, and connected this via Ethernet cable to the Virgin Media super hub, would the Extreme then boost that signal upstairs?
In theory yes. I say that because the Extreme will only repeat the signal it receives from the Express at the bandwidth that it receives it. So the term "boost" needs a bit of clarification. The Extreme should boost the signal strength, but NOT boost the bandwidth. That means, that any clients connected to the Extreme, should be able to connect because of the strong signal, but they may not get decent bandwidth (speed) performance due to Wi-Fi interference between floors.
The best way to test this is to make Signal-to-Noise (SNR) measurements at the location upstairs that you wish to place the Extreme. For this test, leave the Extreme where it is, but use a laptop to take the measurements where you plan on placing the Extreme. If the laptop gets SNR reading greater than 25+ dBm then you can expect a reasonble good chance that the Extreme can successfully extend the Express with enough bandwidth for even things like streaming.
Please check out the following AirPort User Tip for details on how to take these measurements.