New Pond Keeper needs help

I inherited a pond about 2 years ago in my new house in the south of
England and although it was a nice feature I ignored it until very
recently. Its about 3000 ltrs (650 gals) 2m X 3m X 0.7m and I think
its many years old, I think its made of concrete and has some very
attractive stonework around and in the pond, the sides look like stone
bricks. Someone obviously spent a lot of time on it so I thought I
should do something with it. It's in a very shady spot but does get
some direct sunshine in the middle of the afternoon.

It was not filtered and everything was overgrown, it basically looked
like a swap. I started by clearing all the overgrown vegetation and
have left a decent sized patch of Lilly's in the middle with some
oxygenators and reeds.


Then (I didn't now about pond vacs at this point) dredged the bottom
with my bare hands then a coarse net, then a fine net. So now I have a
thin layer of sludge at the bottom. I then added a good quality
filtration system with a small water fall, UV clarifier, an air ball
and some lighting.

I already had three largish goldfish, I've added 3 Tench, 3 Commits,
3 Goldfish, 3 blue orfe, 3 Sebunkins, all are quit small 3in.

Everything seems fine expect I can't see the Tench but I assume they
are ok. The water was very murky but after a few days it has started to
clear (you can see the rocks at the bottom of the pond) and is no
longer green.

My questions a

1 - Have I done the right thing in terms of setup and cleaning?

2 - I have had mixed advice on this but should I drain and clean the
pond and does this need to be done every year?

3 - Can I keep Koi in this pond?

4 - Is there anything else I should do?

5 - How many fish should I keep in the pond?

5 - Where have the Tench gone?

Sorry for the long story but I didn't want to leave anything out but
needless to say but I am now totally hooked!!!

Thanks in advance for you help.