Thinking about this some more, what I actually need to do is as follows:
1) Create a full backup each month.
2) Create incremental backups based on that full backup for the rest of the month.
3) At the start of the next month, make a new full backup, so that the next lot of incremental backups are then based on that.
4) Keep only 3 versions of the monthly backups, so that files deleted from my system will disappear after 3 months.
I don't think this is possible with Ghost. Perhaps somebody can tell me otherwise? Alternatively, does anyone know if Acronis True Image can do it?