welcome
there are people on here better qualified to answer your questions, but my general understanding is:
1) andrew usually turns up for soundchecks, but not always
2) sometimes, even when there, his involvement is limited and it's more about the guitars and backline
3) they sometimes soundcheck odd sisters songs that they have no intention of actually playing live
4) soundchecks are typically closed to the public, but they have been known to invite one or two hardcore fans in if it's a special occasion such as secret, warm-up or anniversary gig - THIS IS NOT THE NORM, however
as Being645 says, soundchecks are primarily for the band to get their equipment set up correctly for that venue. having members of the public in the soundchecks can only ever delay or distract this process, hence it is not usually allowed.
5) having said all that, the band and crew tend to be around the venues for most of the day, and soundchecks can get quite boring, so it's not unknown for them to pop outside for a breath of fresh air.
there are quite a few photos on the web where fans have 'bumped into' the band during these smoke breaks... chris, ben and si are particularly sociable. andrew can be if he's having a good day, and you aren't a d*ck with him
6) the timings can vary, but afaik the crew and the kit usually arrive at a venue in the morning, and then band tend to wander in when the basic PA and stage kit has been set up, which i would imagine is usually mid/late afternoon
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in short, you're unlikely to get into the soundcheck but if you hang around the back door you may bump into them when they duck out for a cig, or a bite to eat.
hope that answers your questions
