Btw It looks like if you sacrifice 45cm on the narrow side of the pond for a filter chamber, you can put in an ultrasieve without trouble. Ideally, this should be on the side nearer the walkway side so maintenance is easy. I did a quick drawing. Will link it up soon.
Maintenance for this sort of system is not back breaking
It involves flushing the crap out of the sieve every weekend or so. You should never have to clean the other chambers very much.
This will mean sacrificing 25% of the pond to the mechanical filter though.