I followed the recipe diligently, but added a bit of extra yoghurt in for maximum granddad compatibility.
It turned out a bit too grapefruit, and a bit too squidgy - it fell apart as I cut it. Disaster! Turns out that getting experimental is fine, but when you've met perfection - and this cake recipe is pretty darn close to it - you shouldn't mess with it!
The granddad visit got postponed, though, so I've no idea if he would have liked it. But the next time I make it, I'm pretty sure I won't be moving even a hair's-breadth from the recipe.