Best answer by woz
If you don't have access to a previous bill just have a look at your meters, with a bit of luck (but not guaranteed) you may be able to see this information actually written/stamped on each of the meters but don't confuse it with the meter serial numbers which definitely will be on each of the meters.
Here is a guide to what you are looking for:
in this case the supplier number would be the "16 6789 0123 222" part
If all else fails there's this:
but I wouldn't bother with the 0870 call part - maybe 0191 216 3000 works? (it's National Grid)