Yes, I did it. I joined aged 22 from uni and would not have qualified for the full service pension at retiring age. Expensive? Yes, but at a time when you are being well paid so can afford it. Worthwhile? Yes, because you are buying into an index-linked pension, a rare and beautiful thing these days, valid for the rest of your life. Believe me, when you are retired, every extra penny counts!
The FPS are excellent. Their advice will be worth acting on.