It looks as if this type of pension is disregarded as income in calculating tax credit entitlement, but the tax credit manual expresses this in a complex way referring to specific legislation under which the pensions have to have been granted so it is not possible to be certain.
You probably need to declare it on your claim anyway, unless the notes with the claim say you don't have to. Would be best to ring the helpline and try to get a definitive answer, but you will need to have precise details of the pension and whatever info. you have about the legislative basis for it.
If they tell you it is not disregarded, don't just accept this - they have been known to be wrong! Go to a welfare benefits adviser or CAB to get it checked out.