IR esa & pension

I need some advice before I act & wonder if you may be able to help as no one at any of the advisory agencies seem to be able to advise me?
I'm 59 and have a serps/s2 pension. I have received my statement & it says that it matures next year in March on my 60th birthday. It also says that I have had access to it since 55yrs of age. I was unaware of this as I have paid little attention to the past statements as have found them quite confusing & they led me to believe the total lump sum was a little over £4000. It now appears that I can draw this pension now which currently stands at £9795 gross or leave it when I l think it would increase. If I wanted to take it as income it amounts to roughly £340 a year.

My question is this:
If I take the lump sum (as could really do with it) would it all be classed as income or only the remainder over and above £6000? Also would they say that I have been overpaid for the past 4 years as I didn't get the pension out at 55yrs old even though the mature date is when I'm 60?

I am loathe to telephone the DWP as have heard horror stories of people that have had their benefits stopped due to notable income, which I don't really understand. This as you can see is not a massive amount of money but the implications of me claiming it as a lump sum could be especially as my area is supposed to be changing to UC soon.
I am on PIP.... IR ESA & Severe disablement allowance of £64 pw

So confused!

Thank you in advance

Kind regards

Thank you