I thought that they kept £100, (under the new rules) and then refunded, at a later time / date, minus the fuel served.
They reserve / ringfence £100, then give change, minus cost of fuel.
Basically they just check you have £100 in your account.
When I got home, checked my bank account, only fuel dispensed £40 had been taken out

They say "this is to help us" control our finances

Just letting you know, in case anyone's not sure, how the system works.