Mercedes Benz C Class starting problems

I have owned the car for 8 years without any problems. About 6 weeks ago I drove 35 miles to a meeting and when I tried to start the car after the meeting it wouldn't fire up. Unusual I thought as it has always started first time. Plenty of fuel and good battery. After trying for several minutes and cranking the engine for about 20 seconds it finally started as if nothing was wrong and ran smoothly. I used the car for several weeks without further problem. Yesterday I drove to Gatwick and on returning to the car it was hesitant to start bud did so and I started the 150 mile journey home. We stopped on the M4 for coffee and fuel and when I tried to start the car the original problem returned and I had to crank the engine for 20 seconds or so before it fired up and ran smoothly for the journey home. The problem occurs now every time the car has been standing for any length of time. My garage is not sure what is causing it and need to plug it in to a 'fault code reader' but thought I would ask if anyone else had experienced this problem. Thanks.