Well, you could try contacting the Bristol Magical Society, to see if they can suggest anyone:
http://www.bristolmagic.co.uk/forms/form.php
However, your best bet, on such a short timescale, may well be the local Yellow Pages.
Although I should warn you that £45 for an hour's entertainment is terribly low, and this may well cause you more problems than finding a Magician who's free on the day.
The usual going-rate is, generally, £60-£100 per hour
Hope this is of some use!