Steve,
I have an unbranded pattern item from 7dayshop.com, link here:
http://www.7dayshop.com/catalog/prod...ducts_id=99219
It doesn't give quite the same life as the Pentax OEM item, but for £6.99 I can live with about 75-80% of the genuine item's capability.
The K10D uses the same battery as that used in the Minolta 5D and 7D, so a bit of searching for minolta NP-400 may turn up more results than you've already found.
I have no experience of the Hanhnel item, but I've used Hahnel rechargeable AA-batteries before and found them to be very good. It depends if you want to speculate £7 for the unbranded or not. It works fine for me.
Regards,
Duncan