Ah, so you were in fact looking for benefit fraud cases, and you had to pick which court specifically to do this, and travel all the way to Birmingham for it (I assume you're based in London).
The point is, you see, that if you had to do that, you then can't do what you did next and extrapolate:
"No idea what the percentage is - but having spent a day (quite recently) at a Birmingham magistrates court where 99% of the cases were concering benefit fraud - amounting to tens of thousands of pounds - and that's one court on one day - then I feel pretty confident in saying it's a widespread and costly problem."
Because by your very selection process you know it's not representative. (NB I'm not doubting that court deals with a large number of such cases, but clearly you're not expecting this level to be similar across all courts or you would have just rocked up at your nearest.)
It's also a bit interesting that you got to spend the whole day and travel to Birmingham for this, just to observe rather than to chase a specific story. I could be wrong, but that has an air of being in response to an invitation by a press office (for the DWP, or the local fraud team, or whoever).
Sorry, I'm not trying to make you out to be some sort of baddy. But one can end up being steered by govt - covering benefit fraud instead of road tax fraud or credit card fraud, or whatever - when a certain route is made smooth.