That sounds like a battery that's reaching the end of its life and you need to sort it soon as a dead battery can swell and damage the phone. Lithium batteries generally have a charge cycle life of 1000 - 2000 time, but it'll be less if you've allowed it to go flat - it's better to top up frequently and there's no memory effect.
Most manufacturers (especially on sealed devices) cover the battery for the full warranty period. You can either contact Vodafone or HTC for a repair.
