sub-DB reading higher than feeder DB

Sparkingchip: 
 

Picking up on the comments by @mapj1 if there’s PV on the roof it is possible the upfront meter could run backwards, so the landlord could be on a winner with whoever pays the bill based on the second meter paying more than the bill for the first meter.

Could fall foul of this? resale brochure.PDF (ofgem.gov.uk)

Sparkingchip: 
 

Picking up on the comments by @mapj1 if there’s PV on the roof it is possible the upfront meter could run backwards, so the landlord could be on a winner with whoever pays the bill based on the second meter paying more than the bill for the first meter.

It doesn't necessarily have to run backwards, just have the PV (or any other generator) connected between the two meters

My advice is to take seriously only what the main electricity supplier's meter is telling you. Whatever other metering there is sounds as if it may be reporting erroneously.

If the main meter is installed by the power company for billing purposes, then that is more likely to be correct, they are experts in this sort of thing and generally get it right.

Mistakes have occurred but are relatively rare, and are very rare indeed on a simple domestic sized install.

My earlier remarks listed various possibilities for error on large, 3 phase industrial supplies, such as wrong ratio current transformers, blown voltage fuse on one phase, current transformer installed wrong way around, etc. These are not applicable to a domestic sized install.

Do be aware of the possibility that the suppliers meter might be in error. Most improbable, but not impossible.