we were there last march from the 16th till 20th,it was quite bright,but walking the streets because of the high buildings made it a little cold,but when you got into the open it was nice,i would say its very similar to what our weather is at that time of year.so you could get all the seasons in one,good luck,have a good time.
It's really variable from year to year. We have had nice mild temps one week and blizzards the next. There is almost always a big snow storm in March here, even at the end of the month. I think you'll have to play it by ear and check the forecasts right before you come. Have a great time!
Try this site: http://www.weatheronline.co.uk/ will give you an average weather based on 9 years results anywhere you want, as well as the current temps and all the rest.