There are a number of such courses offered by various providers. The Expert Witness Institute organised the first couple I went on, which were on writing Expert Witness Reports and cross examination as an Expert Witness.These three courses covered the most difficult areas of the expert's role, though I discovered in doing them that my natural approach to these things is the correct one.
I am presently finishing the writing of a Part 35 compliant expert report for a UK government agency, and I hope that one of the two cases I have been offering expert opinion on will go to court later in the year so that I can have another opportunity to practice assisting the court in person.
One of the issues we covered yesterday was how to challenge the expertise of the other expert to offer an opinion on the matters under consideration. This is something I think it likely I will have an opportunity to practice in future, when there are so many non-engineers willing to offer advice on engineering matters.
Simply put, expert engineers are Chartered Engineers. Non chartered "engineers" are probably neither experts nor engineers, and Chartered Chemists may be expert chemists, but they are not engineers. If you are looking for an Expert Witness in Water Engineering, you want an expert engineer.
There are two engineering disciplines which understand how water and sewage/ wastewater treatment plants work- Environmental Engineers and Chemical Engineers. Chemical Engineers have the deeper understanding of how processes work, and are consequently traditionally a lot better paid (something I am trying to fix with Nottingham's Environmental Engineers). So a Chartered Chemical Engineer is likely to be far more expensive than the miscellaneous scientists and "engineers" who claim to offer expertise in our area, but if you choose wrongly, the court will be unlikely to allow you to change your "expert" once you have chosen one.
At best you get what you pay for, and Google seems full of people who will write you an "expert" report on anything, including my area of expertise. I guess they are gambling on the fact that 98% of cases never make it to court, so they will never be cross-examined on their partisan and ill-argued reports. Sooner or later I look forward to seeing one of them have to defend their dross to a barrister.