All companies are now legally obligated to offer a workplace pension with contributions up to a minimum of 8% of pay for eligible workers (over the age of 22, and over £10,000 per year earnings) . if you are eligible, after a period of postponement (3 months) you will be automatically enrolled. The Union contributes the full 8% for you, so you do not have to pay anything. Further details will be sent out at the time with your Aviva ID, and further information can be found here:
https://www.aviva.co.uk/business/workplace-pensions/