Thanks TIM 50. I'll keep an eye out as I suspect the centres are of a standard size.
Nick - thanks mate. A black one would be best if you've got one. Yes, sold the Atom in May 2012, two weeks before my little girl arrived. She's not 19 months and I had an opportunity to buy an R500 so took it up. Not finished with the Atom chapter yet though and hope to order another one day.....