I found what appears to be a good offer on Quidco...
"£10 Boost Reward | Travel extras | Starts 07 October
Take the stress out of Holiday planning and get all the extras you need for your trip, plus get a £10 Boost Reward when you opt-in and make a purchase in our travel extras hub.
Simply hit the opt-in button, click through, and then make your purchase as normal with one of the following retailers; Europcar, Enterprise Rent-A-Car, Scotrail, Holiday Extras Airport Parking, Purple Parking, P&O Ferries, Manchester Airport, London Stansted Airport Parking, Omio, No1 Lounges, Carrentals.co.uk, RAC UK Breakdown Cover, AA UK Breakdown Cover, Rescuemycar.com Breakdown Cover, Start Rescue, Tesco Bank Travel Insurance, LV= travel insurance, Confused.com Travel insurance, MoneySuperMarket Travel Insurance, Sixt UK, Hoppa, Rentalcars.com, Heathrow Airport Parking, Airparks Airport Parking, Holiday Autos, HolidayTaxis, Booking.com Car Hire."
From what I can tell, as long as you make a purchase that normally activates a cashback reward, you'll get the bonus on top. Does not mention having to purchase a particular type of ticket. I'm a 'Premium' member, so not sure if all accounts eligible. Would appreciate not posting to deal sites for 'likes'. Edit: Scotrail cashback requires you to purchase a ticket wholly within Scotland.