I have been through 3 rounds of interviews with one of the Big 4 consultancy firms. At the end of my final interview I was told that they would like to offer me the job. The interviewer told me that HR will negotiate salary etc and that they will be in touch. 2 weeks passed then came a phone call from HR (a long time in my mind) and we negotiated over salary. Previously I had provided a salary band i.e. 50-55 and told them if they make an offer in the upper middle of the band then they wouldn't be too far off my target. Another 2 weeks have passed and I had expected at least a written offer. Basically should I sit tight or do you think getting in touch and asking whether the offer is in the post would damage my negotiating position? Also has anyone else experienced this sort of length of time from verbal offer to written offer?

Why don't you give them a call stating that you have been given an offer by another company. State that you are their preferred choice of employer, and so are very keen to see what they have to offer. This shouldn't damage your negotiating power.