Yes, any British national is entitled to a State Pension when they reach retirement age. But, most have a private pension as well.

For the purposes of the application, I would include it because it is not harmful to include too much information.

However, I read the question as - does your sponsor have savings (yes, £3500 in your case). Does he have shares? Does he have property? Does he have another source of income - the pension is only really relevant if he is receiving it now because he is retired. That is my opinion.

But, for your ease of mind, include it. Look at it this way

1) You include it. The ECO acknowledges it, and it helps your application.
2) You include it. The ECO has no interest, it doesn't harm your application.
3) You don't include it. The ECO would like to see more evidence of savings, and this would have helped. It has an adverse effect on your application.
4) You don't include it. The ECO has no interest. It doesn't harm your application.

They are the four courses I guess. So, looking at it simply, best to include it.