We bought a new house almost 10 years ago, and Vista's right, when we asked for the sungle sockets to be replaced with doubles (the house was still foundations at this point) Barratts wanted to charge us over £2,000!
Regarding penalty payments, we had a completion date etc sorted, and moved our first van full of contents to the new house in time for the hand over of keys, only to be told that there were no carpets in the house, and we couldn't go in yet. When we brought the rest of our stuff over, we were told they were still waiting for the majority of the carpets to be delivered. We had to sleep in a hotel that night (at their expense I might add).
The following week I had the week off work in order that I could sort the house out, and the builders had to spend a lot of the week doing rectification work such as replacing the bath (which was split) painting and other minor works. All this snagging should have been done before they agreed to hand they keys over!
The Regional Manager offered us £100 compensation, I wrote to the Boss (Sir something or other) and eventually they paid our mortgage for the first month (around £650). They don't like negative publicity!
So read the small print of your contract, and if you can, get some money off them as compensation.