How to maintain old pond filter with no drains?

A couple of years ago I moved in to a UK house with a pond approx 16ft
x 10ft x 3ft, complete with no instructions.

The water flows from the pond via a side-pipe (a few inches from the
floor) in to the adjacent filter area, where it's pumped back to the
pond using an argonaut J75 0.5hp external pump (3000gph).

The problem I have is that a lot of books/sites I read seems to be
about ponds with bottom drains and easy filter drainage, and my pond
has neither.

The filter area is a large (semi-covered) rectangle and has two rows
of brushes, and some submerged material in bags before the brushes;
but it doesn't seem to clean the pond at all: it's always very murky
or green. I tried lots of floating plants last year and have also
tried water changes.

There are a few small koi and goldfish in the pond and they seem
healthy and have survived the last couple of years.

So, my question is, what's the best way to maintain this type of
filter and clean up the water?

Should I be replacing the bagged material and brushes? Buying
matting? (and presumably putting it so all water has to flow through
it?) Buy a proper pond vacuum and clean out the filter? Should I be
cleaning everything lots?

I can see lots of things I could try, but don't want to waste money.
Assume minimal diy skills!

Any info/sites would be useful since most of the information I see is
about the biotec style of filters.

My other question is about piping. The pump has a 1.5" outlet, but
the return pipe is 1.25", so the pump is wasting energy. I assume to
upgrade the pipework, I need to buy pressure piping and solvent weld
rather than just buying cheap push piping from the local diy shop?

Many thanks in advance,
Ian.