It can be quite hard to find an MPR on a customers bill. There are so many other reference numbers and meter serial numbers on a bill. The MPR contains only Numbers.
If you find a reference number of a bill and it contains letters this is probably the Meter Serial Number.
Here is where the MPR is on a British Gas Bill
You can see on this invoice that they have placed the MPR on the right hand side half way down page 2 of 5 within the literature.
This can be a difficult one to find as it blends in with the rest of the non important information.