Eurostar appear to have quietly updated their 'snap' offerings: https://snap.eurostar.com/
Before, you only had the option to pick the day, now you are able to select 0600-1300 or 1300-20:00 slots each day.
Example from the 19th of March:

(Photo above shows time slots of 06:01-13:00 (with a price of £50 single) and 13:00-20:01 (with a price of £40 single) on the 19th of March.)
This should hopefully make the service more attractive to those that need a little bit certainty!
For those that have not used it before, be aware the tickets are non-exchangeable, non-refundable, non-transferable and non-cancellable.
Before, you only had the option to pick the day, now you are able to select 0600-1300 or 1300-20:00 slots each day.
Example from the 19th of March:

(Photo above shows time slots of 06:01-13:00 (with a price of £50 single) and 13:00-20:01 (with a price of £40 single) on the 19th of March.)
This should hopefully make the service more attractive to those that need a little bit certainty!
For those that have not used it before, be aware the tickets are non-exchangeable, non-refundable, non-transferable and non-cancellable.