Pilot16 - just check your figures on that discount. a 5% discount for 20 hours equates to one free hours lesson, so £250 is a bit over the mark. Did you mean £125?
No experience of block booking for discounts - all I would add is that, as distaff says, you get a better deal if you plan to do the 20 hours in a short time (because they don't have you money much longer than they would anyway), and it's less risky as they're less likely to fold in, say, a few weeks as apposed to a few months.
Whatever you do a credit card is a MUST.