So you have your diploma level 2? To be honest, you don't need a massive portfolio to get up to level 2 NVQ from where you are. You just need a bit of work experience with someone who has the NVQ2 or equivalent. Not easy to find, but the hardest bit really is finding a workplace recorder. If you work as self-employed and have a plumber friend who could watch you work and sign the paperwork, that should also be okay (it's a bit unusual, but get your assessor to check the rules).
Usual portfolio for NVQ2 would be two cold appliance installations, two hot appliance installations (a bath, sink, etc can count as a hot OR a cold, but not both) , and two central heating installations (add a radiator to a new location), all conducted in site conditions. Other less common options are two new soil stack installations, and two new gutter and drainpipe installations. You have to cover a choice of three of the five options, twice each. It has to be your own work and own design. You need to ensure that each category is carried out on TWO sites (so you can't change two radiators in one house and call that two). There are also some maintenance tasks you need to do to qualify, and these need to be witnessed by another plumber, but the tasks themselves aren't hard to find: tap washers, cleaning a sink trap, that sort of thing. You need plenty of photos of you doing the work.
If your gas guy is offering you gas training, ask if he can help you get your level 2 NVQ at the same time. It may be what he is offering by the sounds of it.
