Composting plastic

[Ishouldhavechosenagoldfish]

I have just noticed on a cardboard sandwich wrapper that it is compostable as the cellophane-y bit 'constitutes less than 13% of the total'. Our council tell us, however, to tear out the windows from window envelopes before putting them in the compost wheelie bin. So what is the difference, and is 13% the magic proportion of non-compostable material? So if 13% of my wheelie bin consists of, say, roof tiles, is that OK?

That seems really misleading to me. Did it mean that the cellophane is 'real' cellophane and will break down in the compost heap? There are two types - the proper stuff made from cellulose which will compost and plastic man-made stuff which needs to be landfilled. - Where was the sandwich from? I would write to the manufacturer and ask them to be less confusing!