It's not made obvious at all, but it is possible to record the Red Button service (on Sky+ - don't know about Virgin).
On the Sky Planner, record the speedway at 6:00pm, as you would do normally.
Then (somewhat bizarrely), record the Spanish football that follows at 6:55pm.
Only then doe the planner give you the option of either recording said football OR the Red Button coverage of the SGP.
Hopefully that'll help those (like me!) who'll be at speedway on Saturday evening & want to watch the SGP later.
Cheers,
Bob