Underage wizards and magic use

[Dom McDermott]

No: HPFGUIDX 48812

I was wondering if anyone knew any more about the rules etc for the use of magic for non-graduates? If I'm reading it correctly (and forgive me for being unable to provide quotes to back myself up) 'underage wizards' (presumably those who have yet to graduate) are not permitted to use magic outside of school. 
Does this mean *all* magic? And also how strict are they on these rules?
In COS, Harry is reprimanded by letter for using magic in the pudding incident. This letter comes to him even though it wasn't he who was the cause, so how do the MOM target individual underage wizards? Did they note that magic was used in the Dursley house and as the only wizard in the house, Harry must be responsible? If so, what would happen if, say Ron used magic in his house when his parents and older brothers were there? Would the MOM be able to ascertain that the magic was from Ron, or would there be the presumtion that it came from one of the qualified wizards (and witch)?
Another note on this subject is that two other occasions stick out in my memory. Firstly, one the train to Hogwarts in PS/SS, Ron is trying to turn Scabbers yellow. Elsewhere in the series I believe it mentions that the students are still able to use magic while onboard the Hogwarts Express, but to me it gives the impression that Ron has perhaps tried the spell before (ie: when not on the train) and also, around the same time, Hermione arrives and says that she has only tried a few simple spells, but they've all worked fine for her.
Secondly, during the build up to the quidditch world cup in GOF, there is an incident where a child uses his fathers wand to engorge a slug, and the mother gives the impression ('Havent' I told you not to play with your father's wand?' or words to the effect) that its not the first time. 
Do the parents of this child receive letters from the MOM about improper magic use, and has Hermione already got a stain on her magical record before the first day of term at Hogwarts??

I know I've possibly ambled on a bit more than I should have, but its one of the wee things thats been bugging me :)

 - Another BTW, and rather much on a tangent, is just a comment on rather coincidental timing. If I'm not mistaken, Malfoy, Crabbe and Goyle Seniors knew each other from their DE days, tho I'm not sure of any evidence that they were necessarily in the same year at Hogwarts etc. Is it not strange that they should all have sons effectively in the same academic year? And in the same year also as James and Lily Potter, and Arthur and Molly Weasley!! (although with the latter its not so much coincidence, as they seem to have made a sport out of it ;-)

I'll ramble away again now, hope you've all had a good Christmas (or are still having one)

Dom
http://www.underagewizards.co.uk

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