The /0's were done before they entered service in may with WMT
Indeed they did. What I was trying to imply was that there was simply no slack in the WMR fleet at Tyseley before the 172/0s came and saw off the 150s from Hereford work, and the 153s from NUCKLE. (The 230s saw them off from Marston Vale services, but that's beside the point.)
The main point is that there are now more units available on depot, so WMR can afford to send the 172/2s (and thence the 172/3s) to Ilford for refurbishment.
Even then, the 172/0s weren't fully done - they aren't yet fitted with plug sockets & WiFi "domes". I assume a further trip to Ilford will be required for them once the other 172s are all done.