LEAVE ON REDUCED OR NO PAY
If the period is 30 days or less, you must pay your normal contributions, so there is no effect on your pension benefits. If you are off for more than 30 days you can either pay contributions to cover the whole period, or pay nothing.
You have 30 days from returning to decide and the cost is your normal contribution rate of the pay you would have received. If you pay nothing, you will not build up any membership for this period, and this will affect your pension benefits.