I've not looked at this forum for a while, so my apologies if I'm posting old news, but the following is useful information:

When the new UK flat rate pension comes in in 2017 it will be necessary to have 35 years NI contributions to get a full pension, not 30 as at present.

Many ex-pats will have got a statement of their contributions when they went abroad, and been told they have more than 30 years and enough for a full pension. If they reach pension age after 2017, they will now need to top up their contributions to get the full amount.

Currently topping up, by making Class 3 contributions, represents good value, £2.70 a week to buy an extra £4.11 a week pension (£140 a year for an extra £213 year).

Information on how to pay here: http://www.hmrc.gov.uk/ni/volcontr/abroad.htm

The other aspect to take into consideration is whether or not you have been 'contracted out' while making contributions in the UK.

If you were part of an occupational pension scheme and were 'contracted out' it is possible that even with 35 years contributions, you would still not get the full rate of the single tier pension.

Here is a fact sheet with some details about the new pension.

I think you mean Class 2 rather than Class 3 as Class 3 is more expensive than 2.70 GBP/week.