Thanks guys. Will try the 'manual rotation' idea....hope it works!
I've only had this tank for a couple of months, so not yet had to change the bottom filters, but so far (according to water parameters), things are OK. On the other hand, one of my mates has a Rio 400, and has just finished ripping out the Juwel filter in place for externals, for the same reasons as yusufosman! Soon find out I guess.....