Hi Dumplumb
Let me try and put this into some kind of perspective for you. The current 6129 is being replaced with the 6189, but not all centres are up to speed with the 6189 because C&G have extended the 6129 and 6089 until 31/7/12, so a lot of centres will still run this course until they have to change. The 6129 is broken down into 13 units, 12 theory and 1 large practical unit covering 60 assessments, then you have to go on site and complete the 6089 which is another portfolio of evidence. This needs to explained in detail when people register so you understand whats involved (and yes it does carry another cost). With both the 6129 and 6089 together, this gives a level 2 qualification.
The 6189 is broken down into 10 units to be carried out in workshop and 2 units on site. If you hold the 6128 tech cert it is transferable across to the new qual, but only gives units 1-8 because sanitation and rainwater systems have not been covered. You still have to go on site to complete the final 2 units which will give you the full level 2 qualification. If you can find a centre that runs the 6189 then you have a chance of completing your training to become a plumber. If you want to become a heating engineer only, the 6128 is still transferable without having to do the sanitation units and only 1 onsite unit is required.
The problem will be trying to find a centre willing to train you on the final elements, but good luck with your efforts, hope this helps