UK Fly fishing queries (newbie)

I'm taking up fly fishing and have a few queries and I'd be grateful for
any tips or advice and recommendations of good fly fishing venues (I'm
in Bromley, Kent) good fishing websites, books, etc.

I'm completely new to fly fishing, I have done some course fishing, but
that was about 25 years ago and for fun rather than serious fishing. So
I'm really starting from scratch.

A friend at work is an occasional fly fisher and he has kindly taken me
out (to Bewl Waters off the A21) and borrowed me his gear and shown me
the basics, re casting etc, and he has given me some advice, but I don't
want to keep pestering him all the time, and I'm not convinced about
some of the advice he has already given me.

So my questions are...

Can you put fish back after catching them?

The reason I ask this is cos my work mate said you always kill your
catch and take it away with you, but I thought some venues only allow
you to take a certain number of fish, (to preserve stocks) and if this
is the case does that mean you have to stop fishing once you have caught
up to the limit?

Or can you carry on and just put back any further fish that you catch?

What if you catch a fish that you don't want to keep? (eg: non
trout/salmon).

Do I need to have a mat to lay the fish on when I'm unhooking, etc? I
know that carp fishers and course fishers use them, but I'm not sure
about Game?

What does the number on the rod/line mean?

I know you have to match the line to the rod and I have a 9' (2.7m) fly
rod that states it should use 6/7 line, and I have some 6/7 line but
what does 6/7 mean?

I've been told its best to use floating line in summer, and sinking line
in winter, is this correct?

I assume it is right cos flies will be on the water during summer, and I
presume during winter the grubs, larvae, etc will be below the surface
so I can see some logic in it.

I have a selection of dry flies. I presume these should always be used
with a floating line cos they are intended to stay on top of the water?
Is this right?

Therefore, I presume its best to use wet flies during non summer months?

How do I keep dry flies in good condition? do I need to use the
floatation fluid stuff and coat the flies occasionally? if so often do I
need to coat them? (eg: every time I use them?)

Can I use a fly rod AND a course rod at the same time at the same
location (not sure whether my fishing licence allows this?) I have a one
year non migratory and course fishing licence.

Do you get many venues where the above is plausible (eg: trout/salmon
and other course type fish in the same place).

Lastly, I'm not really sure about fly lines and suitable knots!

I know I need a backing line, and connected to this is the fly line, and
then connected to this a thinner leader line to the fly. But what is the
best method of connecting these?

Someone told me to just tie knots, and someone else told me to use
v.thin rubber sleeves to cover joins (with or without superglue?)

What is the easiest recommended method?

Is it worth taking more than one reel with different line types on each
reel, say one floating and one sinking, so you can swap on-site?

Many many thanx for any answers you can provide...