There are some common themes coming out of the missing money saga on the PO's Horizon epos system. First of all, £10K seems to be the average missing amount. Second, subpostmasters' mutterings about their federation's role in this are getting louder. Where is the federation in this?

And third, the PO seems to be in denial and there is no real chain of command so that retailers can appeal to an area or line manager. One of them said: "We don't even know who our area manager is."

Another said that, at a recent regional meeting, Horizon was the main subject.

However, there are glimmers of hope. Tom Hedges, who trades near Skegness, had losses which climbed to about £10K. "I persuaded the powers that be to change the central processor (the brains) which sends info down the wire. Everything's gone okay since, but I haven't got the money back."

And Bill Morrison has had a meeting with the PO at which the rep instantly put her hand up and admitted a problem. Bill, whose Budgens and petrol station trades in Ramsgate, Kent, was also experiencing losses of about £10K and was personally shut down, although allowed to appoint a stand-in. His theory is that it is cock-up rather than conspiracy.

Other retailers have suggested that it may be human error ie the subpostmaster entering amounts in the wrong column.

John Middleton from ShopEase, a company which specialises in epos, sent me a couple of possible explanations.

He says: "Can a computer system get its sums wrong? Yes. Any software grid of columns and rows (be that a database or spreadsheet), which performs virtual calculations, can fail to correctly write its deduced answer. This could be caused by disk write failure, data being erroneously stuck in a cache or memory pocket, data corruption or simply not written or calculated at the assumed moment in time.

"But and this is a big 'but' we software writers borrow practices from Dickensian times and provide safeguards by way of 'double entry' bookkeeping. So if the accountant's clerk makes an error, his books won't balance at the foot of page. In simple terms and without software parlance, this means that critical calculations are performed twice (or more) from different aspects while employing entirely separate processor internal instructions. Two rows of figures (in individual tables) for example, could be summed on an accumulation basis or as a whole, both virtually and retrospectively.

"The software then has to find a way of informing the user, as a matter of urgency (and before deficits of hundreds of pounds arise) that all is not as it should be, and/or running a triggered or automated self-diagnostic/repairing procedure. Whereas I have no personal knowledge of the Horizon system, I cannot imagine that this would not follow similar safeguards. And I would have expected any 'phantoms' to have haunted users and made their presence felt!"