Personally I woudn't bother with a return flexible time ticket - they are about 4x the price of a standard ticket with which you can get on any train within 2 hours of your booked one (subject to space). I think it also gives you access to a lounge or something but as you're only on the train for 35 mins and, the whole attraction of Eurotunnel is the speed with which you can be through the terminals and out on to the autoroute, what's the point of that?
The price does go up as you get into summer though (Eurotunnel adjust their prices based on demand) so yes it is better to buy now than to wait.
If you collect Tesco reward points, you can use these for or towards a Eurotunnel standard tickets at 3x value (i.e. £40 of reward tokens for a return crossing costing £120)
Is it a good idea to think about booking the Eurotunnel now for a trip in the summer holidays? Do the fares go up as time goes on? We're thinking about a return flexible time ticket. Thanks in advance.