Gosh, the gruesome bit's all over in MUCH less than 30 minutes unless your BP's incredibly low - and by the sounds of things your BP is likely to skyrocket when you're in there, you'll have the bag filled in 30 seconds!
Let the staff know you're nervous, they're real pros and will put you at ease.
I think the best thing about the sessions here is the atmosphere - everyone turns out in force to the local hall to donate every time the bus comes round, and there's always good crack over juice and biscuits afterwards. The horrible bit isn't that bad (it's just a little cannula, not a huge great weapon!) and really won't take long