Which is cheap & best bulk SMS service?

The problem with other providers...
With many providers you buy "credits" from them and they sit in your "account". As you send messages each message deducts the credits from your account until you need to buy more. The problem here is that you may have 1000 numbers and only 970 credits. These companies force you to buy another 1000 credits or more - just to send your message.
Some providers even expire your message credits if they don't get used and you can end up vastly out of pocket if you don't keep using their services.
Other providers also charge credits per messages at a rate of 1.2 - 1.5 credits per SMS, which can make sending messages very costly indeed when you think your paying 4p and its actually 6p!.