Hi Matt
I am doing the update course at the end of the month and was told the requirements needed for the one day course, so below is a copy of what was said. Fortunately i only did the 16th 2 years ago.
PLEASE NOTE: Delegates will need to show that they hold a 16th edition City and Guilds 2381 qualification (June 2001 onwards) or an electrotechnical qualification such as 2356/2360/2330 (June 2004 onwards); In all cases, we will need to see either an original certificate or by a City and Guilds enrolment number
Hope this clarifies things for you.
Cheers
Craig