fop-pdf-images and caching

I'm using fop-pdf-images to load an external PDF to use as a background on
all pages. Works great most of the time :)

However I've noticed a couple of things around caching. It might be my
setup, but maybe someone can point me in the right direction before I start
stepping through it.

First, I get a hit on the external PDF for each and every page. I've had a
look at the PreloadPDF and it looks like it's meant to be caching, but
apparently not. Is there some configuration I'm missing for this?

Second, if I rename the external PDF I get an error as expected. However
renaming it back doesn't seem to be picked up immediately: it takes one or
more runs before it's found again.

Note that I've got an EventListener hooked up to throw an exception on
(most) resource error events. Could that be short-circuiting things?

Thanks for any help

-- 
Matt Kynaston