Ethernet setup advice wanted

>What is a RJ45 economiser like this one What is a RJ45 economiser like this one

Ethernet uses 2 pair (4 wires). Standard ethernet cable has 4 pair which means that 2 pair are used, the other two are not. This device most likely will split the the cable into two 2-pair connections. You can do the same thing without this device by splitting the pairs when you're terminating the cable.

>Are there any pitfalls of using an ethernet cable extender much like you can with a phone cable extender?

Yes. Connectors are a weak point in the link. The fewer the connectors, the better. Length is a greater issue though. 100 meters is the max. If you go over that length, all bets are off.

>Is RJ45 just the connector type

Yes.

>Again, why so many types of ethernet cables.  This 20m length is nearly £5  whilst this patch cable is only 50p   whats the difference

Can't say, they're both the same link. :-o

I would get a spool of cat5e cable, some RJ-45 wallplates RJ-45 connectors and a crimping tool. Put in runs from your central point (where your internet connection is) to the different rooms. Then terminate the ends. If you want to get fancy, you can put in a patch panel at the central point.

10base-T and 100base-TX use one pair for transmit and one pair for receive (4 wires total). Standard Cat3, Cat5 and Cat5e cable has four pair. So most cable runs have two pair, that while they may to terminated, are not used. It is not unusual (in an economy type install) to pull one cable and split two pair for one connection and the other two pair for another. In my house I do that on the cable run to my office. In the other rooms of the house, I split the other two pair off for the telephone.

Now splitting the run will prevent you from doing POE (Power Over Ethernet) and 1000base-TX (gig ethernet over cat5e up to 100 meters).

As for speed, Cat5e can do up to 100mbps up to 100meters.

When you split a cable as you've described, you are NOT splitting the signal, like telephone (POTS) or cable TV.  You are splitting an 8-wire bundle into 2x4-wire bundles.  Each 100BT connection is separate from the other.

100Mbps over copper is faster versus "125g" wireless.  There is significant drop in signal strength once you get 3-5m and farther away.  Even if both were reading "100Mbps" the wireless link has significant latency.

For instance, I have a 20Mbps downstream link.  Over wireless 54g at 4-5 meters, one large download was reading 2hrs.  Switching to 100BT made it 20-30 minutes.

Get the wire in the walls first.  Terminate later.  Make a home run from each location to a central point.  Don't worry about if you have too many cables.  There's no such thing as too many cables.

The cost to run additional once the walls, floors, and ceilings are closed up is so much higher you'll be kicking yourself in the rear.  You'll also need to learn how to do drywall patching and texturing...and it still won't look the same.

It's not uncommon to run two Cat5e cables to each room.  Cost is nothing for materials.  You can even leave the spare in the wall for future use in audio-video for NTSC, HDMI, VGA, speakers, RCA jacks, etc....all of which can be run over that Cat5 cable.

Buy in spool boxes of 1000ft each.  Depending on your supplier and the grade of cable, it will run from $50-200 each spool.