While I am working at home I have been provided with a Yealink T42S VOIP phone, do I need Talk Talk to provide a phone number for me or must all this be setup with a 3rd party leaving Talk Talk to just help configure my router to allow the VOIP traffic ?
Talktalk won't provide the VOIP account or a number, that's down to your business to provide you with that setup information - quite surprised they haven't
Configuration of a VOIP phone usually requires setup of an account with a provider - I use VOIPCHEAP for my own IP Calling, but if its a business supplied phone, then either they preconfigured it before sending it to you, or need you to configure it.
Might help you in configuring the phone. Whoever sent it to you might need to assist you with settings/configuration/account setup.
As for talktalk, there is a SIP ALG setting on the router. That may need to be 'enabled'. That usually allows VOIP through. I don't use Talktalk's router, my routers SIP ALG setting is enabled. That's usually enough without port forwarding/triggering required.
It's more likely that you will have to set SIP ALG to disabled rather than enabled; it depends how you're planning to receive your incoming calls - either by SIP URI, or by E164 phone number.
If you're not planning to receive any incoming calls, and are only intending to make outgoing calls, then it doesn't really make an awful lot of difference whether SIP ALG is enabled or disabled.
If you're planning to receive incoming calls on a SIP URI, then you will probably either need your ISP (TalkTalk) to give you a static IP address, or you will want to use a dynamic hostname service (such as DynDNS or No-IP); you will also want to set your router SIP ALG to enabled. This has the effect of telling the router to convert your internal IP address into your external address on your outbound calls so that your return calls can get back to you.
On the other hand, if you're not expecting to use SIP URI, and are instead expecting to use an external registrar that gives you an E164 phone number (such as sipgate or Voipfone), then that service will use registration signals that your phone sends out every few minutes in order to figure out where to send your incoming calls, and you should therefore set SIP ALG to disabled.
As for how you configure the phone - you may have been given SIP registration server details with username and password, or alternatively you will have been given a provisioning server address.