We are about to sign up for broadband for the first time with BT and get a BT home hub for wireless broadband connection.

We only have one phone socket.

Can anyone tell me:

1) is this one socket where the hub will be connected?
2) do we use the hub to make landline calls?
3) will our additional handsets (dect phones not bt) which are plugged only into electric sockets still work?

All this is alien to me so would appreciate any help/advice!!! 

Not quite.

You will need to plug your existing DECT phone base into the phone socket on the filter that is provided with your hub.  It will then continue to work as normal.

You can elect to also answer landline calls via the hub either using the hub phone (if you have one) or another landline phone (could be your DECT base) plugged into the phone socket on the back of the hub.  This has nothing to do with Broadband Talk, which is in effect a separate line into your house that can only be answered / used with a hub phone or other phone plugged into the phone socket on the hub.

If you choose to, you can make landline calls via the hub phone or other phone plugged into the hub's phone socket, but first you have to dial 5.  Otherwise you will make the call via the Broadband Talk service.

To further confuse the issue, the hub is actually a DECT base - the hub phones are DECT phones.  If your existing DECT handsets permit it, they can be registered to the hub as a base instead or as well as the existing base, and then used to make / recieve broadband talk calls. 
You achieve the same end by leaving your DECT phones as they are and plugging your DECT base into the socket on the hub, but if you do either of these remember about dialling the 5 first to use the landline for making a call.  Either of these options may result in some strange ringing patterns, with phones connected to the hub continuing to ring for a while after a landline call has been answered using a landline phone.

Hub phones (the proper ones) will show landline calls answered using a landline connected phone as missed calls.