Drayt - pressed post instead of preview. Let's try that again after a typo check ...
The general advice above is good - restart your pension now!
But a lot of the specifics (employer contributions, doubled years) are from people who maybe don't know the details of the current NHS pension.
So yes. Rejoin the pension. But read up on what you're getting and how to make the most of it. If you're on Facebook join the NHS Pension group on there.
Your old pension will be in the 1995 section. You can't add to that - the number of years in it now won't change, nor will the final salary used to calculate it. It has a retirement age of 60. When you retire you will get (1/60th of the best year's salary of your final 3 years before you left that pension) times (the number of years you were in it). If you worked part time for any of that, it will be calculated on full time equivalent salary but the number of years will be reduced. You will be paid that amount (increased by inflation) every year for life. You will also get a lump sum when you first take the pension. If you don't take it at 60, it doesn't increase and you may not be able to claim back the years you missed.
You may read things about special class status or the McCloud judgement (especially if you join the Facebook group). Neither of these will apply to you so you can ignore them.
When you rejoin the pension now you will join the 2015 section. This is a defined benefit pension, but not a final salary one. Every year your pension entitlement is increased by 1/54th of your earnings for that year. When you retire all those 54ths will be added together to determine your annual pension. Again it is paid every year for life, and increases with inflation. There is no default lump sum (if you want one you can give up some of the annual pension in exchange for a lump sum). Retirement age is the same as state pension age. If you don't take it at that age it is increased and you get a larger yearly amount when you do claim it.
If you want a larger pension, firstly check your state pension entitlement on Government Gateway and make sure you are on track to get full state pension.
You can then look at ways to increase your NHS pension.
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Additional pension. This buys a set extra amount (up to £6,500 a year) on top of the 2015 pension. You can pay for it in a lump sum or in installments over a few years. It will be paid as part of the 2015 pension and can only be taken at the same time.
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AVCs - this buys a private, defined-contribution pension alongside the NHS one, but as a separate 'pot'. The terms and conditions are different. It will probably be worth less money than putting the same amount into the NHS one, but is more flexible because you can take it at a different time.
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ERRBO - this does not give you a bigger pension, but allows you to take the 2015 one at 65 or 66 without the usual ppenalty or taking it early. If you want to take advantage of this option you need to start making ERRBO payments as soon as possible after starting the pension because it only applies to pension accrued while making them; the portion of pension accrued beforehand will be reduced for taking it early.