The impact should not be overly surprising, if you have been paying attention to attendance at the park. It has been, and still is, shockingly low.
Almost everyday of the week right now the major attractions are all walk on, including weekends and sunny days. A busy day right now, seems to consist of 20m queues for Spinball and Rita, and everything else walk on. A quiet day you won't even find a queue at any attraction.
Here is the average queue time for the major attractions posted by Alton Towers for the past 10 days:
Monday and Tuesday last week were slightly busier due to the number of end of term school trips. There were even a few school trips boosting attendance on the Friday and Wednesday, But at the weekends, and even now with the the summer holidays starting from Thursday, you can see attendance remains rock bottom. Indeed it would seem since the incident school trips have been the only major driver of attendance at the park, as they were likely planned and moneys paid before the incident took place.
It will be interesting to see if attendance slowly rises over the summer, particularly if we get a period of sunny weather.