Generally speaking, the ride queues tend to be quieter in the evening as mazes will absorb an hour of people's time. What I would recommend regarding fast tracks is look at ride times via http://ridetimes.co.uk/ or the Alton towers app. The 12th, 13th should give you the best idea about queues for the dates you are going, so just check a few times during the day like morning afternoon and evening. If the queues look busy, it might be worth getting fast track. This crowd calendar predicts queues https://queue-times.com/parks/1/calendar Also consider that weather plays a big part. I'm expecting it to be quieter this weekend as the forecast is rain, but a sunny day can bring extra people out. Seems like most English school kids will be off from 19th, but it varies from school to school. Scottish schools have broken up by then too, so this will add to crowds. Generally I'd say it's going to be busy, but I think you'll have time to do everything, especially over two days as you plan. Remember also it depends on your willingness to queue, I don't mind queues 30 minutes and less, you might have a different tolerance.