When you say block, better be a big block. I've got �1000 worth and have only ever got �150 in 30 years.
I'm only keeping them because it's an insignificant amount (well it isn't, but they're paid for if you get my drift) and one may come up. Not holding breath tho!
I agree with dzug,
The likelihood of winning (regularily) is so low it's not worth having.
Of course you have to bear in mind that you get NO interest with Premium Bonds,so your capital is always depreciating!