You will rarely get 60 out of any combi unless your running a very low flow of water through the taps.
Setting a combi at 60, doesnt make the water come out the tap at 60, it limits the maximum temperature to 60, the actual temperature depends on how warm the cold supply is and how fast you run the water, most of the vaillants are sized and restricted to give roughtly a 42 degree rise in the cold water temp to hot water, so for 60 degrees you would need about 18 degrees in your cold main coming in, which is probably about 16 more than you will actually have at this time of year.
Get the thermometer out, run the hot water, (give the boiler a couple of minutes to have hot water running steady at tap) and watch the temperature, if its fluctuating up and down significantly then yes, there may be a fault in the boiler (or just as likley in your plumbing somewhere) but if its giving a steady temp, which is in the region of 35-40 degrees above that of your cold water, then its probably working exactly as its specified to do