Error message on body module. No errors on airbag module.

I tried a proxy alignment but the light is still on

You mean the airbag module? Your body computer probably can’t talk to the airbag module. Go to parameters setting I think it is, that will show you if it is even seeing your airbag module.

You can buy modules on eBay for about £100.

Yes. With multiecuscan I can read and test the airbag module. It's even visible when testing proxy alignment.
If i buy one module, is simply plug and play? Or it should be reprogramed? I know it must be crash error free...