If you do not mention the M&S vouchers then you don't automatically qualify for them iyswim.
If you order a package online then you immediately get £100 M&S vouchers, however it is not the same if you order a package over the telephone.
During my conversation with the Sky man last night I mentioned the M&S vouchers and he told me that the offer did not extend to people over the telephone, BUT since i mentioned it I qualified for a £50 credit on my bill.
I hope someone finds this information useful.