Sounds like a lot of money, or is that over three years? If three years at full cost (no Government funding)it's probably about right. 2357 is not an NVQ, it's a Level 3 diploma which combines knowledge units (equivalent to the 2330 or 2365 technical certificates)and performance units (equivalent to the 2356 NVQ). To complete the diploma you need to be working on site at least part time because most of the evidence for the performance units must be gathered from real jobs.
Another possible route for you is to go for the 2356 technical certificate (cheaper than 2330 as there is some Government funding for adults, 2330 only funded for 16-19 year olds)and if you are working on site enrol on the 2356 NVQ, if you're not on site you could carry on with the 2356 tech. certs. & have these mapped across to 2357 knowledge units & do the performance units when you get work in the trade (2356 registrations end in December this year).
It's probably worth talking to your local FE college, but hurry, they'll be looking to fill the last few places as term is just starting.