No nothing has changed. The requirement for "attendance" sigs for Trainee to Track remains at 15.
There is indeed a split - which is a
minimum of two days. So you could do 13 Track and 2 Flagging, or 13 Flagging and 2 Track. Or anything in between, say 10 Track and 5 Flagging.
On top of this you will be required to have completed the correct modules for training (in this case it is Fire, Flag, and Basic Marshalling).
You also need to do two assessment days, one for Trackside and one for Flagging (these days are ON TOP of the 15 attendance days, not part of the 15). An assessment has to be signed by an Examining Post Chief.
Note - you have to have done the minimum attendance days and the training day BEFORE you go for assessment.
I think your confusion around a split of 5 days comes from
Track to
Exp Track. There the min number of attendance is 20, with a 5/15 split.
Here is an link to the MSA requirements - though this particular document does not mention the split.
https://www.msauk.org/assets/upgraderequirements.pdf
Ok - so that is what it is formally. More prosaically, as Graeme suggests, it is always worth doing more, and be in no rush. Don't upgrade until you are good and ready. A good measure is would you be happy to have a trainee with you and be prepared to make a decision on your own about going trackside if you have to - you never know when you may be remotely posted on your own in this age of low numbers.
Hope that this helps?