I think someone said somewhere it was 8 of the existing 153 fleet - 6 PRM and 2 PRM lite, with the 4 active travel units. Thought it was mentioned that the 2 lite units were also to be converted to ACU but I may be wrong there.
Sounds about right. From what I know the PRM lites won't be converted but those along with the ACUs will have their small toilets re-activated as there's now a solution to getting the tank for those which I believe Scotrail have done.
I see 175109 is on the Cheltenham line today. Last one in service?
109 is booked:
10:30 Cardiff to Cheltenham
12:35 Cheltenham to Maesteg
15:17 Maesteg to Cardiff
There's also another 175 diagram in the evening, currently not allocated to anything:
18:29 Cardiff to Cheltenham
20:10 Cheltenham to Cardiff
22:35 Cardiff to Gloucester
Ecs to Canton
That's not yet allocated so may or may not end up as a 175.