In fairness to Merlin, this is a price rise of just under 1.3% and means they will be getting 48p extra from each full priced ticket.
The increase is well below the inflation rate, and Merlin could argue that in real terms, the price of a park ticket has come down.
With the minimum wage increasing each year, and new pension requirements (Not sure if it applies to part time staff, but will apply to full time staff) there is an additional cost that the park has to cover.
I think its also realistic to expect the materials required to maintain and refurbish the park rides has increased.
When you think about it like that, it only seems sensible that the park increases ticket prices, to cover there additional costs, so investment in the park does not get reduced.
That all being said, when paying full price at Alton Towers, I do not think you get value for money, and when paying £90 for a family of 4, while a fun day out can be had, I am sure there are plenty of places a family can have just as good day, but only spend a fraction of the cost.
Ian