Tip the driver?

My wife thinks I'm rather tight with cash. She's convinced I should investigate my Scottish routes (no offence to our friends north of the border, nothing to do with me).

Travelling back from a smashing meal at our favourite restaurant, we gave the usual hand-signals to hail the old black cab. The taxi-driver took offence, so we had to wait for the next one.
Anyway, we boarded the next taxi and gave instructions to get us home.

Having parted with a crisp new £10 note, I did not have the remotest inclination to part with more. The look on my better half's face told me that further action was, indeed, required.

"Tip the driver, then, Mike"
"Really?", I painfully replied.
"Yes" she said, with a pinched expression, leaving the cab and heading for our front door.

"Driver, ................... plant your rhubarb early"

At which I made a hasty retreat.

No cocoa for me that night.