How can I extend wifi connection on BT hub 3 using airport express? Staff at MK shop told me it was possible so I bought one!

Unfortunately, everyone on the "staff" is wrong....or possibly they all misunderstood your question......if they told you that the AirPort Express could "extend" the wireless signal from a BT Hub using wireless only.

You must use an Ethernet cable to connect the AirPort Express if you want it to provide more wireless signal for your BT network.

Go get your money back. You would be doing the store a favor if you ask to speak to the manager to bring this to his attention, so that other users will not also be given incorrect information by the "staff".

A belated answer but it may still help others: what I discovered after 10 months of frustration and just before having BT to shift my fibre optic line due to frequent loss of signal and slow connection... is that their business Hub 3 (which is actually the same device they give to the Home users...) is totally inadequate (unless one lives in a studio flat with all devices being near the Hub) and utterly useless as it cannot be used as a wireless extender either. My configuration ( large semi-detached house with as many as 6 devices using the Internet simultaneously) is as follows: BT modem connected via Ethernet to WAN port of the new Airport Express (small square one), creating a network. Older AirPort Extreme in another room extending the network (very easy to set up with the Utility --just connect to the same Network and keep password the same too using WPA/WPA2 security settings). Old Airport Express in the lounge as a further extender, connected via Ethernet to an Ethernet Hub which in turn is connected to a smart TV so it can go online.

I get at least 10 times the speed of the BT Hub at the furthest point from the main airport --around 12/14 Mbps where I had 0.2/1.5 before-- and a lot better than that elsewhere, with no loss! And nobody can use some of my bandwidth anymore (BT Hub is set to create Open WIFI so inevitably some of the bandwidth will be lost --read the small print on the contract...).