I had a similar experience some years ago. 4 months elapsed between the day my decision form was received by the administrator, and the date shown on the insurance company investment certificate. The amount of the investment was substantial, and the chosen funds had increased in value by just over 10% in the 4 month period.
I sent the administrator a short sharp shock letter, asking them to either compensate me for the difference, or let me have the name and address of their PI Insurer. I subsequently sent then 2 very sweet (not) reminders at 7 day intervals. All correspondence by registered post of course.
I received a revised investment certificate from the insurance company 3 weeks later showing an additional contribution, roughly equal to 10% of the initial contribution.