I usually buy the fitted sheets from the cot/mattress supplier. Our cot is from Mamas & Papas and I bought some elsewhere at first, which didn't fit at all - so I think a lot of companies only do the covers for their own cots.
To the original question - sorry no answer for you but, we have both of these cots: https://www.mamasandpapas.co.uk/search/?query=amelia&category=All and judging by Bethanie's size at 14 months, she should be alright.