Chris J Dixon wrote:

Until they have suitable meters, which many suppliers are not yet
willing to fit, the amount actually exported will simply be
estimated. Some expect this to be set at 50%, which is probably
high for most installations.

Who can blame them? they're already being stung for the 41p * "n" units,
why pay for an extra meter when they can pay 3p * "n/2" instead? Even
with the optimistic n=1000 a few people have quoted, then it's £15/year
to not fit the meter v.s £100's to fit one.