Hi joe, it all depends where the mackeral are, that may be 50yrds out or 150yrds you just have to try your luck. we don't fish in the bournemouth area so can't help you with what beach, sorry. we normally go to chesil, maybe someone closer to you could help with best times and tides for your area as well.
I agree with the evening being best, particularly from 2 hrs before to 1 hr after the high. You should not need to go more that 30 yards out, but that bit further will enhance your chances. In Cornwall the Macs tend to shoal in June and the break up arround now and so chances os success increase, but bag sizes drop.