Hello buildersmate,
Like you I did not trust our beloved Pension Service to accurately calculate any SERPS, S2P benefits. For that reason I Contracted Out as soon as I could, at least that way it was under my control. I Contracted Out to "with profit" schemes, both of which have demutualised, giving me demutualisation benefits. I did contribute many years ago to a superannuation scheme, which will apparently grant me a whole 6d (old money) per week.
I contracted back in about 5 years ago, when advised to by an IFA. I am sure you are well of the reasons why I did that, Goverment guarantees on pensions, shrinkiing annuity rates etc.
At the end of the day I doubt if there is any way you can calculate your benefits personally, even an actuary would be stumped on this one. We wouldn't need the Pension Service then would we? You can of course apply for a Pension forecast, but I am sure you know that. Sorry to take so much time to give you a non-answer.