The short answer to your question Rosie, is that your GP does not know because nobody knows; the research has not been done.
Nobody knows what bacteria are effective, what dose is needed, and which conditions benefit etc.
There are some papers which suggest probiotics may help in acute diarrhoea, and a few other conditions...
http://summaries.cochrane.org/CD003048/probiotics-for-treating-acute-infectious-diarrhoea
...but on the whole hard evidence is missing.
By the by, I sincerely hope your doctor is not a nutritionist; as Dara O' Briain says, a nutritionist is to a dietitian what a toothologist is to dentist.
The big companies like Danone already make millions selling their products , so why should they bother to do the research.
The European Food Safety Authority (EFSA) is tackling this problem so they may eventually be forced to back up their claims.
http://www.guardian.co.uk/society/2010/oct/19/efsa-rules-probiotic-health-claims-unproven
http://www.nutraingredients.com/Regulation/EFSA-beats-off-more-Lactobacillus-probiotic-claims
http://www.independent.co.uk/life-style/health-and-families/danones-actimel-does-not-alleviate-diarrhea--efsa-2155887.html