I have downloaded and am testing Syncback now. Looks to be a great tool and does make the backup of my other files, mail data etc a fair bit easier and less prone to human error.
I could never figure out why my 2 x 2Tb drives never had the same available space - a simulated run of this with Mirror settings showed me a load of hidden photo files I had removed from one drive and somehow they remained hidden away on the second drive. It can be hard to find miss matches when there is 1.8 Tb of data to try and compare. Thanx for the tip on the software team