@Lilmo I've had a look at your bike specs. The brakes do indeed look crap; and I'm a huge fan of cross levers, so two good purchases. In fact, I'm just realising after 2 1/2 years that I should probably try to stop relying on the cross lever brakes - I'm not sure I could ride a bike without them!
Then have a read of this
http://road.cc/content/review/100555-shimano-sora-3500-groupset
They say the cassette is comparable to Tiagra, which I have on my bike. I once had a gear cable replaced at 3am in the Scottish highlands** by a volunteer who had only ever worked before with retro down tube shifters, which aren't even indexed (yeah, it's a long story
). Within an hour I had my 10 perfectly indexed gears to get me back south.
Your bike is a bargain, but it is not a cheap bike.
Evans are a decent bike shop and they want you to come back and back. They want you upgrading, servicing, buying lots of kit, buying you next bike from them. They want you to love and trust them. So they have a process where your bike is checked by a mechanic before it leaves the store. Clearly there was a snafu and that didn't happen. A lose headset is more than an oversight, it wasn't looked at all. These things happen. Take it back there if you can, tell them you haven't been able to get the indexing right since it was new and demand they sort it out. If it needs a new cassette (which I doubt) they should replace it. Oh, and while you are there, talk to the manager. He'll*** be very concerned to hear that a bike left his showroom in that condition. You could have sued them if you were in an accident due to the fault, and I imagine Evans would replace all management in that store if that happened.
My final list of 2 bikes was a Cannondale and the Giant I bought. Cross levers (which you now have) and red paint made the decision for me. It's a very good bike. You are really going to enjoy riding it.
** ok, technically it was the lowlands, but it was very lumpy for a southerner.
*** it's almost always a man. Let us know if it isn't.