Exit Life Through Window

I've been offered the possibility of a really great job over the summer. It's not definite, I have to go and train for a week or so first, but after that I could do it. The problem is, I'm playing lots of gigs with The Promise this summer (the band I'm in for the Scout centenary). Literally, I'm going to need about 4 weekends off and one whole week, and maybe more. My current job, the charity fundraising thing, is really flexible (I chose what hours I work each week) and so that wouldn't be a problem. But with this new job I think it would be. So do I go for the new job and stress about conniving my way into weekends off, or do I email the guy back and tell him that I've been offered the chance of developing my music career and I think it would impinge (is that a word?) too much on the job for me to be able to take it? Grr, it's so annoying, the job I've been offered is tour-guiding round London, which I really want to do. But I just am not sure it's going to work. Ick. Advice would be very welcome!

( In other news, I got locked in the toilet yesterday for half an hour... )