thinking of using an up graded alternator,as planned upgrades include central locking electric Windows and may be heated windscreen,would I need to upgrade the wiring to the alternator ?
If you are still running the original wiring loom, then you should upgrade the wiring regardless of whether you are installing new kit or not - the old wiring was crap and underspecified when it was installed, and i'm pretty sure that old insulation doesn't improve like old wine...While you're at it, get an autosparks to put in a proper fuse/relay system if you are going to do any serious upgrade works.
However, to address your actual question - adding central locking adds only a 'momentary' load which normally occurs when the car is not running, electric windows adds only a limited and intermittent load, and a heated windscreen only adds only a few Watts, so the alternator and alternator wiring upgrade is not theoretically necessary unless you are looking to add a high load which will be constant - like big driving lights, high-demand sound system, electric seats/heating/air-conditioning system or the like.
The principle is that the alternator runs the 'normal' running load such as radio, lights, vehicle electrical demands etc and recharges/float charges the battery while the car is running, and the battery provides the capacitance to store or discharge energy outside the 'normal' range, for instance to cover the power dip or surge when you turn your lights on or off. If you've added heaps of 'stuff' to the electrical/Electronic load, then you need to measure or calculate the demand on the alternator whilst the car is running and fully loaded (electrically) and compare that to the wire size to ensure the cable isn't overloaded to avoid the risk of fire in the wiring loom.
Is that too much for one post?