Hi guys bought a new sill off izzy,just wondering how big a job it is to replace the old slightly rusted one.Not doing it myself as dont have the expertise and knowledge off how to do it,just curious.

Big job to do it properly. To do it 'properly' you really need to drill out the lower spot welds on the rear quarter as the sill tucks up under the rear quarter.
The quicker way is to cut out up to the rear quarter and the corresponding piece of the sill and weld it on.
The front half is pretty straightforward, the back half is a little more involved.

I did my sills when I removed the rear quarters.... Was very easy.. Like shoota said you can unpick the lower half which I strongly recommend if you have rust in you sills you will want to treat it before putting the sill back on... No point doing half a job, if you want a quick easy fix bog it ;)

yeah its in the front off the sill,near where it curves up.nah dont want to do that ,the car is being stripped back to bare metal so mays well fix everything will it in that state.