Q: What is the best way to extend a Virgin wifi network in a thick walled house?

The "best" way to extend Virgin (or any other) Wi-Fi network, where you have "thick" walls is by Ethernet with all of the wireless routers/wireless access points configured for a roaming network.

The next best way would assume that using Ethernet would be impractical or undesireable, would be to use Powerline adapters to create a pseudo Ethernet connections between routers but still using a roaming network configuration.

The last method would use all wireless connections between routers. This would have the least chance of success due to the builidng construction preventing a quality signal between routers which would be a requirement. Also, this would, most likely, require that you use networking equipment from the same manufacturer for compatiblity. In your example, the AirPort Express could not extend the wireless range of the Netgear router over a wireless connection between them. Instead you would need to substitute Netgear's solution to do so.

Apple's extend a wireless network feature works like the hub and spokes of a wheel. The "main" AirPort Express would be the "hub", so it would need to be located in a central area in the home. Other AirPorts....located at the end of separate "spokes".....could extend the wireless signal.

Each extender must communicate directly to the "hub"....not to another device at the end of another "spoke", so your idea of a "chain" to extend the wireless network signal would simply not work.

Since you have thick walls, it is also unlikely that a wireless setup....even with devices placed at optimum locations....would allow the network to perform well.

The best solution in situations like this is always to run Ethernet cabling to other routers from a main router. This way, devices can be located in the exact areas where they are needed to provide more wireless coverage.

Ethernet powerline adapters might be an option if you cannot run the Ethernet cabling.  In theory, you already have the wiring in place, you just need the adapters.

In practice, things do not always work as well as the theory might suggest.  The bottom line with devices like this is that you cannot know how well.....or if.....they will work until you try them out in your home. Older wiring may not accommodate these devices.

For these reasons, you would want to understand the store's return policy to test a couple of adapters to see how well they perform at various locations around the house.

If they work well during your "tests", then you can purchase more adapters and add AirPorts or other routers to set up the network the way that you want. I am not necessarily recommending that you use Apple products. Virgin might have some solutions that would be much easier to install and configure.

Having one of the IT guys come out to survey your home and offer some suggestions...even if you have to pay a fee.....would be very wise thing to do before you proceed with any potential plan.

The IT guys may spot a potential problem.....something that you have not thought off yet.....that will help you avoid delays and heachaches in the future.