We are currently doing 4 full backups on 4 Fridays in a month. Can I reduce it to 2 full backups fortnightly without affecting restores at any point?

Carillion wrote: We are currently doing 4 full backups on 4 Fridays in a month. Can I reduce it to 2 full backups fortnightly without affecting restores at any point?

I think you are asking how your restores might be affected if you were to do fewer full backups and more incrementals?  In general, you will need more save sets in order to completely recover your files if you have more incremental save sets between fulls. 

There is a wonderful description of this in Chapter 4 of the Administration Guide for NetWorker, "Backup Groups and Scheduling."  It's well written and quite thorough.  Maybe it will help you understand.