They are often right and seldom wrong - they are mostly good guessers because of their experience. The danger is that if you call in this heating engineer and it isn't the pump after all - he may change it anyway - so have him or anyone else, demonstrate (point out to you) just where the leak is before replacing anything. Leaks are often easy to spot - you can see the water dripping out.
Remain close while he works - make him a cup of tea - ask him what he's doing - engage in the work - ask the question, "is that a guess or are you 100% sure?"
If he's any good he should not mind your close attention and after all it's your money paying for his services.
Good luck...
Peccavi, January 2010
