Originally on Orange. Had it unlocked. Currently using EE SIM.

Running an old version of Android and bloated with the Orange crap. How do I;

A) put a "vanilla" flavour of Android on? Do I need it to have HTC Sense (whatever that is)?

or

b) put a newer version of Android on? Currently on the very old v2.3!! What's the newest that the hardware will support - 4.4?

You need to look at the threads over at XDA unless someone here is running a modded Desire S. The Desire S was supported officially by Cyanogenmod up to CM7 i.e Android 2.3 Of course other custom roms will be available and these will be more relevant.

This phone is very very old now and you will be flogging a dead horse. Time to move on I think.