Based on the way you describe the problem and the fact that you have had a few visits by Gas safe registered people!!, it may be an idea to call out the manufacturer but a couple of things to consider. In my opinion, if the pipe work and radiators are sound, i.e no leaks, you may have more than one problem giving the same symptoms. Losing water while the appliance is operating(heating) could be the result of a faulty or dirty pressure relief valve, an uncharged, weak or faulty expansion vessel, a good indicator of a fault associated with the vessel is to monitor the pressure gauge after about half an hour operating in central heating mode and see if the pressure increases to about or above 3bar. It should always stay below 3.
If you continue to lose water from the pump looking thing, as you describe it, then you should get your engineer to check the o ring seal around the cap. Usually where you see the leak is not always where it originate from, so a thorough examination is very important. Unfortunately, not all registered people are boiler repair competent. With the above information you could probably have a discussion with the next person who will visit and ensure that they will work to a plan. It may take a couple of visits to resolve the problem and the longest time on site I feel should be about two hours. The colour of the water maybe an indication of how dirty the system is and perhaps lack of inhibitor, not necessarily related to your problem. Hope my input will help you understand things a little better so you may get the right person to resolve the problem. Wish you every success.
Shocks, January 2010
