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The beloved character, played by Alison Steadman, will play quizmaster on Thursday (21 May).
With questions all about the show, the online quiz will start at 8.15pm and will be in aid of UK charity Marie Curie, which helps care for and support those suffering with terminal illnesses.
You don't have to donate, but if you're in a position to, those who do will be entered into a draw to win a prize.
Visit the Marie Curie Facebook page and click on interested, then the page says to do the following:
The cast of Gavin & Stacey have been vocal throughout the coronavirus pandemic, with Nessa giving us some words of wisdom a couple of weeks back.
Ruth Jones, who plays the character, took to social media to tell people to stick to the rules of social distancing.
In Nessa's typical no-nonsense style, she told everyone to 'stay out of my way' as she runs her 'half marathon around Barry'.
Nessa said: "Oh, what's occurring? Not a lot, other than a global pandemic but the question was rhetorical.
"Now listen, I am not here to give advice, there's plenty of others that would do that for you. It's your life and I am not about to tell you how to live it. I wouldn't do that to no one. I wouldn't even tell myself to live my life.
"If you see me in the morning doing my daily run, my half marathon around Barry, don't even think about breaking that two-metre rule because if you does, I will not hesitate to tell you quite clearly to back off.
"Because at the end of the day, when all is said and done, no word of a lie if truth be told, just because you don't feel ill don't mean you're not infectious, you could be riddled."
Nessa's speech to the nation comes off the back of the huge success of the Gavin and Stacey Christmas special. The one-off TV event was watched by 18.5m viewers when it aired on Christmas Day last year.
That runaway success has sparked talk of a reunion, which could see James Corden, Mathew Horne, Ruth Jones, and Rob Brydon brought back together in some capacity.
If the Christmas special is anything to go by, you'd have to say the demand is there.
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