I, too, am starting at St. George's in September and I only started volunteering in August last year (in a hospice a few times a week), as I only made my final decision that medicine was the right choice for me (after lots of procrastination) in June and it took ages to get my CRB back.
I didn't think I had much of a chance, but thought it would make a good 'practice' year and so gave it a shot anyway. In the St. George's Information Booklet (
http://www.sgul.ac.uk/dms/52B0D910FF...7D8516DFC3.doc) it gives you details on scoring for work experience and, although you can only get the maximum (5) for over a year of experience, from my reading you can get 4 for less as long as you can show at interview what you've learnt.
So, there really is no harm in trying and you can use your GAMSAT score for 2 years.
Stephen