LGPS - Buying Lost Pension

Cazza

I've recently returned to work following Maternity Leave and am applying to purchase the amount of pension I have "lost", following the time I was off and not paid.


I can choose to pay this either by lump sum (payroll deduction or direct payment) or regular payroll deductions. I am opting for payroll deduction, as I don't usually complete a tax return and this method means the tax relief will be dealt with automatically.


My question....
The lump sum deduction would be £498.62
If I opt for regular deductions then the amount is different....
12 months - £40.09 per month / equivalent to £481.08
24 months - £20.54 per month / equivalent to £492.96
36 months - £14.03 per month / equivalent to £505.08


I can choose longer time periods too, but the cost increases each year.


I am a basic rate tax payer and expect to remain so for the next few years at least, so spreading it out won't mean the difference between higher and basic rate tax. In 2018/19, due to Maternity Leave, I expect my taxable income to be £12,506, so the lump sum wouldn't mean I'm below the tax threshold completely.


Payroll / pensions haven't been able to tell me why there's a difference, so I can only assume it's maybe indexation / inflation


I'm not sure if it's better going for the monthly deduction or lump sum? I'm inclined just to pay the lump sum and get it done, but if it's a few £s cheaper to spread it out and I understand why, then I'll do that. Thoughts (and explanations!) welcomed. TIA.