John in Hull Mmmm, tricky one this....
Have often been in sessions, with the odd tape m/c lurking around. Shouldn't be a problem, but, always best to check with whoever is running the bash. Recording Concert performances on the other hand brings up all sorts of copyright issues. Anyone wishing to do this should seek out permission from the artist concerned prior to the gig.....Most people I know (inc me) are happy enough to waive our statutory rights, but, I have known occasions when Artists have refused to perform, just to protect their rights....!!
On the other hand, if many people hadn't boot-legged Nic Jones' performances all those years ago, I wouldn't have any material to work with now!!!
Generally, courtesy, sympathy, and communication before a gig is probably the best way, and if an artist says NO, they probably mean it.
Hope this helps. Cheers Ralphie