Has anybody else had a problem of a banging noise followed by a grating noise when switching the AC on???

I have had 2 AC compressors on for this problem now

Sounds like you need another one then :)

sick ofgetting them replaced

I get it replaced and its fine for 2 months and now its gone again. noticed it before it got damaged. You can actually feel the judder through the pedals also

Are you running it for at least ten minutes every week? If you don't then it can cause the compressor to seize.

gets used very often

Is the tensioner ok on the aux belt?

as far as i am aware yes

Taking into work and they can fix it

Need the master cylinder done on the clutch aswell anyway