No news is bad news, but we knew that - the fact they've made every effort to publicise each milestone in the Stadler deployment means that they're definitely making use of every opportunity to promote the new stock and correspondingly, probably means they're covering for lack of progress on the 720s. That none have been sent to a GA depot to start testing, or even to my knowledge been seen on Derby's own test track means they aren't ready yet, as an entity, never mind the individual units. That's most likely due to the software issues first experienced on the 710s. I'm fairly confident that when they're sorted on the 710s, the 720s will continue in earnest. As it stands though, there's no point in manufacturing units that you know will be of no functional value and sit gathering dust for months waiting for a usable software release. I'm sure a few issues may well be thrown up in testing of the 720s as they have already been with the 755s, but we aren't at that stage yet.
As soon as a 720 is released for testing, there will be plenty of fanfare about it from GA I'm sure. Currently though I don't expect the first unit to be sent out to GA for testing until the Summer, maybe June/July time if the 710 issue is actually dealt with fairly soon. If that drags on further, add the relevant number of months to that. In terms of revenue-earning service, it's pretty much guaranteed not to be until 2020 now. If the mainstay of your fleet is over a year late, you don't make a song and dance about it. You change the entry into service and keep quiet hoping nobody notices. That the Stadler fleet offer a means of delivering to a reduced extent on the 'new trains in 2019' statement is a real blessing.
This isn't an indictment of GA per se, it's what any TOC could be expected to do in this situation. Before too long the other TOCs that have ordered Aventras will need to start doing the same. It took a full 18 months for the Class 345 fleet to achieve a fleet-wide reliability level around that which is expected of a new unit's fault-free mileage testing.
The first class of a given product is bound to take longer than the others, but given the 720s have the newer, clearly even less stable software running on them, I don't anticipate the fault-free mileage period being very quick either. Certainly I'd be surprised if the full fleet was fully introduced by the end of 2020 as that'd give them about 10-11 months to deliver and commission all 111 units.