Brake stone guard and cv boot question?

Hi there folks a bit of advice needed. Excuse my ignorance. The wife was complaining of a terrible metal against metal sound coming from the golf. when i looked at the front wheel I could see a bit of metal rubbing against the wheel. When I touched it it just fell into my hand. It kinda looks like a half a rear brake backing plate. When i jacked the car up you can totally see the brake disc, on the other side of the car the disc is  covered by something. Couldn't see anything in the Haynes manual to tell me what it is. I'm assuming it's some sort of guard to stop stones getting trapped in to the brakes. Please could someone tell me what it is? Wouldn't imagine you would need a photo of the bit that fell off, with your experience but it can be provided. Is this going to be easy to find? Also I take the whole wheel hub would have to come off for it to be replaced?
                         This leads me into next question when the car was jacked up i could see the outer cv boot had a huge hole in it, think i've caught it before any damage was done. Even though the job will need to be done at the garage I know from experience that it's cheaper to supply own bits to be put on. I looked at gsf site and seen the cv boot kit outer that i need. I would imagine that contains all the bits and bobs and the grease. Could anyone enlighten me on what a cv boot and cap is? Actually i know what the boot is? what the cap aspect? Sorry for what might seem daft questions, I'm just learning about cars. Cheers in advance

Ahh the rusty disc back plate available from  crazy quiffs not too tedious to replace for a good garage if you don't feel up to it, I would try to get in and spray some plusgas or similar penetrating fluid onto the bolts a few days before it goes to the garage as it will make the job a lot easier for them and hopefully a bit cheaper for you! be careful not to get any on the disc though :-)

Not any available from Crazy quiff for 4-5 weeks unfortunately. Is it a danger to run the car with out the brake plate there? . I would imagine they are there for a reason. How does it go on? Would it make more sense to take car of the road till the brake back plate is available can go on at the same time as the new cv boot. If i manage to get my hands on a second hand one, i assume it would benifit from a wee blast to get the rust off and a good paint. All advice gratefully recieved

One of my brake guards corroded away and started rubbing inside of me alloy 12months ago, ripped it off and still haven't replaced it. No issues since and mine's a daily driver, didn't even get a mention when it went for MOT ;o)