Update ... I've made some progress although there is still a discrepancy. I think that I'll have to talk to the pensions board as suggested.
It turns out that annual payment is considered to be paid at the end of the year so the formula becomes (X * 1.06) + y. This is not clear from the statement document. The calculated figure is closer to but not exactly the quoted figure. When I extrapolate it to 20+ years of investment, the figures diverge quite a lot, as do the figures that have costs included. Not there yet...