Hi John
The 6189 has both on-site & off-site assessments (college based) if you are unable to complete the on-site ones you can not get the qualification so it is a real risk.
The level 3 is a real step-up from the L2 tech cert as you would expect, it goes into a lot more detail (sometimes far to much & off track from work that most plumbers do) but on the whole it is a good course if the teacher knows there stuff. It is normally completed in around 2 years the first year at college is the core subjects :- Cold (lots of water regs), Hot (including un-vented) Heating (design, heatloss, pump sizing etc), Sanitation, (design AGD & RW Systems), Electric's (largest subject area 102 guided learning hours. PS - your know than most sparks if you can pass the exam LOL) & Organising Resources - simple one but exam is mental. Each one has an on-line exam & there is a project exam for C,H,Htg & sanitation. You then have to choose a route for the second year which is either gas or re-newables, to do the gas you have to be working, doing gas work under a GSR engineer & build up a log of many days, otherwise it's the re-newable such a solar HW etc.
If you can do it you come out with water reg's & un-vented tickets & if you can get the gas work experience you can come out with the same as the CCN1 & apply to GS to go on the register but it is a very big ask I think with out the work experience running along side the college stuff (not just the gas).
Put it this way, the course was designed for people who are working in the industry & to stop, so called training centres selling it to anyone who could pay.