Refactor investment and appraisal code

I didn't finish the job of refactoring the appraisal code with #1199 as I wanted to avoid merge conflicts with other work in flight.

Once that work is finished we should have another pass at this code to tidy it up.

Review the investment and appraisal code after the work tracked by #1199 is finished. Start by examining the changes and remaining merge-conflict concerns from that work, then identify what still needs tidying. Done means the follow-up refactor is complete without conflicting with the other in-flight changes.

Written by the indexing model from the issue text.