Sonos doesn’t transmit Bluetooth audio directly at all. Connect a suitable third party transmitter to your Port.

Thanks for that ratty, excuse my ignorance but what is a third party transmitter?‍♂️

esinkin Bluetooth Audio Adapter for Music Streaming Sound System

There are of course better ones which take a direct digital feed from the Port.

If your goal is to watch TV using the headphones, my preference would be to use audio directly from the TV. Passing TV audio through PORT will compromise lip sync slightly.

Operationally, passing PORT’s Line-Out to a Bluetooth transmitter will be the simplest because both TV audio and SONOS audio are easily available. With some “boxes” and operational “grit” you could have native TV audio output or Line-Out sent to the headphones.

I don’t know what you mean by you can’t justify buying the Sonos Ace. However, IMO the best way to get BT music streamed to your Sonos is by purchasing a Roam 2 and your choice of BT headphones. You’d start the BT stream to the Roam2 and group in your other Sonos speakers. I’d recommend that BT headphones support BT 5.1 or BT 5.3.

The OP evidently wants to stream from Sonos to BT headphones. 

I’m with @buzz: if what’s required is TV audio just connect any BT headphones direct to the TV. With any modern TV lip-sync should be perfect as the TV compensates for BT latency. Via Port the audio is likely to be at least 250ms late over SBC, though maybe half that if an aptX-LL transmitter/headphone is used.

For non-TV audio from Sonos, a BT transmitter connected to Port will be fine. 

On 2nd thought you’re probably right

Thanks your for help everyone, my main use with the BT headphones would be to listen to my vinyl from my turntable which plays via Port so am I right in assuming that I need a BT transmitter to do so?

Again, please excuse my ignorance and thanks again for your advice.

Correct. 

If you want to be relatively future-proof go for the FiiO BTA30 Pro. It’d take a direct digital coax connection from the Port.