Unexpected NI bill

I received an unexpected bill for NI this morning

27.30 - 8/10/06 to 6/1/07 711.90 prior to 8/10/06

I've been with the same (FTSE-100) company for more than nine years and worked in a police force for seven years prior to that. I'd expect that both employers would be paying the correct amount of NI for me. Shouldn't they have ?

The NI office is closed and I can only think of one thing that this could be about.

In the early to mid-90s I also did self employed work (writing software). I completed the income tax returns and paid the tax but also I was advised at one stage by the IR that I needed to buy NI stamps as well. I bought about

100 worth. Then someone else from there told me that I didn't need to buy them so I stopped buying them.

Could they think that I owe them for this ?

If I was paying my employees NI through my full time employment, were they wrong to say that I had to buy the stamps anyway ?

Are they likely to be wrong to ask for this money ?

I've just returned to work after five months sick leave (on full pay). Could it have anything to do with this ?

Thanks