You state that you still want to still receive incoming calls from your talktalk line and make outgoing calls using your SIP account OR talk talk as you choose. In that case you will need an analogue ATA, or a router with VOIP capability built in, with PSTN (lifeline) support. For this I use the zyxel 2002-L and for me it works well, but set up is not for the faint hearted and may not even work depending on your router.
Have a look at the Sipura 3000 which is supposed to be quite good, and also the Grandstream HT 488. Both of these work as you will want.