I wouldn't plant more than three, myself, Greytiles. I've got to ask, though: is this going into a heated greenhouse? In an unheated greenhouse, it probably won't bloom; and left outside, it will probably freeze, in the UK.

Curious question to ask at this time of year! In an 18 inch, I'd use four, unless you're adding other plants as well, such as an upright fuchsia or something, in which case you'll only need 3. Should be done mid May ready for hanging outside at end of May... Surfinias are summer bedding plants.

Massive thank you I am planning next years baskets
and have never tried Surfinias before;
Will e mail you next summer with the result
Best Regards Geytiles

I agree as I have 16" and 18" baskets and would put in four.