HI i have also just put a reservation on a new build, myself and my partner both went around the site the first time and saw the house we really liked but were unable to put an offer in until i had some movement on my property, at the time the seller said there was interest in the one we liked and they were just waiting for people to sell their homes before proceeding on this new house.
Anyway after 2 weeks me and my girlfriend were in a position to go back with regards to making them an offer, they initially offered 5% deposit and stamp duty, but after some strong negotations she finally said " what would it take for me to reserve today " so i asked for 12.5% deposit, stamp duty paid, £4000 towards floor coverings and granite worktops in the kitchen, after a bit of huffing and puffing she made a few callls back and said we could have it at that offer, i still wanted to walk away and think about it to make sure i had eveything covered but i got caught up in the moment with my girlfirend and her mum so we agreed and signed.
Now after thinking about it i think i could of got even more out of them, after signing she informed us that a lot of things in the showhouse are optional upgrades which has dampened my mood towards the place as they are very expensive to upgrade and the standard options are not as good as the upgrades.
My advice is tell them what you what and get as much as you can out of them ( when we first went round i could of sworn she said they would pay solicitor fees, but as soon as we signed that offer disappered!!!!! ) so make sure you get everythng written down before you sign and hand over a reservation fee, because your bargaining powers go as soon as you sign.
Hope my story helps you get a better deal.
Martylaa