Exactly how much work needs to be done to the house? Do you know - why not bring a builder friend along to view it. Then you can assess how much you'd need to spend to bring it up to the standard you need.
Don't rush into anything. If it's only been on the market for a month, yet you say 'you keep coming back to it', you can't have been looking that long.
You sound in a hurry, please, please be patient. There's a huge difference between 300k and 340k -
how long would it take you to save 40k? Wouldn't it be nice to get the house for 20, 30 or even 40k less?
Your bidding plan is playing directly into the Estate Agent's hands. You are
doing exactly what he hopes.
There may well not have been any 330 bid. (Isn't it peculiar that people always bid using multiples of 10 or 5?)
And he's expecting your call today with an offer of 340k. And then over today and tomorrow, he'll work you up to 348 at which point the offer will be accepted.
And that's all fine if you can afford that, and have another pot of money held back for work on the house, for the deposit, and have stress tested your ability to repair your mortgage if interest rates rise considerably over the coming years.
I'd be urging you to bring a builder to take a look (even if you have to pay someone). Meanwhile read my Key Post for making Offers here:
http://www.askaboutmoney.com/showthread.php?t=123191