When I accepted my PhD my supervisor cheerfully told me 'you have now aggreed to give up your life for 3 years.' This gives me the impression that only 150 hours a week commitment will get me through. I have a 1st class hons degree and got 84% in my diss. Do you think i could get bye with 40-50 hours a week hard work?

Yes. In fact for most people an average of an 8 hour day will be normal for majority of their PhD. But, like other jobs you will find that the work doesn't always stop when you go home - on occasion there may be papers to read and most of the time you'll be thinking about your PhD.

You may also find that the structure of your working day is governed by experiments, rather than meal times. Most PhD students work very flexible hours, long days are followed by short ones.

See the 'How much work?!?' thread for other comments