Well, because of that slightly confusing 5-20 shares or 20 upwards thing (it's discussed well on here, and I KNOW that 20 shares is perfectly fine), I decided to hedge my bets against any change in that and buy 21 shares.
20 would have been £68.98 (as being me, I noticed the price tended to drop slightly at lunch time, so waited until then to buy!), but in the end paid £72.44 for my 21 shares.
I am much chuffed. The danger now, I've wanted to get into buying shares in things for years, and nothings ever got me started on getting an account etc... slippery slope lies ahead!