If you want to avoid the queues then book tickets ...

If you want to avoid the queues then book tickets online AND book a tour 7 extra/person. Booking tickets online doesn't mean you skip the long queues. Arrive at 9am. Park next to main gates (~ 20/day) then do bag security. (Fast queue by first gate). Avoid queue for desk A. Turn right by security and go to first right hand side building and look for little sign saying 'tours'. The tours are great value. They last about an hour and allow you to skip all big queues. Sunday 23 July the queues were hours and hours long.
Note: Children under 18 get free entry but you need a seperate tour ticket for over 10s. Also need seperate ticket (different payment system) for special garden water fountain days for children over 6. This made getting tickets for two adults and two kids (aged 9 and 11) tricky!