Hi Joe,
The additional units 019 & 020, along with the Environmental exam are obtained through your local FE College or C&G approved Training Centre. The cost is approx. £900. You should be 'working' with a qualified Plumber, carrying out the full scope of the work required ( choice of five systems: hot, cold, sanitation, heating and rainwater) pick three of the five systems and carry out two install of each. Plus various maintenance and decommissioning. Your appointed assessor will visit you in the workplace, to carry out various assessments including Health and Safety assessments. This process will of course depend on your work type schedule, but is usually achievable in three visits.
You will normally complete an 'E' Portfolio to record all mandatory evidence.
Hope this helps
Regards
Teabreak