Rossi_uk --
What you are describing is reactive hypoglycaemia and you may need to have a six hour GTT before they find it. I have diabetes and reactive hypoglycaemia and that's a difficult combination... when my body does manage to produce insulin it doesn't always know when it's produced enough and I can go from having a prolonged high to a low in a matter of minutes.
My GP didn't believe me until she asked me to come in for a fasting blood test and I had a hypo in her office. At that point I was given a meter and asked to keep a record of my lows. Everyone is different and I had hypo symptoms at around 4.2 on the meter, technically too high for a hypo but not if your blood glucose drops rapidly. A three hour GTT didn't find anything odd, a six hour test clearly showed a high followed by a low rather than leveling out like it should have for a normal person. Be persistent with your GP and don't be afraid to print off information from reliable sources like the Mayo Clinic website to take to your next appointment.
Diet is usually the best treatment and you should eat the kind of foods that take longer to digest. Whole grains, complex carbohydrates, proteins, limit refined sugar and simple carbohydrates because they'll cause a hypo as easily as not eating. Most people eat six small meals a day rather than three and keep sensible snacks handy at all times. Best to avoid a hypo rather than using sugar to resolve it because with reactive hypoglycaemia you will have a rebound hypo. If you do need to use sugar then make sure you have some sensible food with it.