Once it was finished, I did a video and it looks like it should do, so it's been sent on it's way. All being well, it will fit, otherwise it's onto plan B!
The good news is, the other covers have arrived in France, and the T cushion is a perfect fit! That makes me happy. The others are going to have to wait until their visitors have left to be fitted, but I'm so grateful she tried one so she could ease my mind.
As far as I know, there are no more seat covers to do in the near future. I think the next big job is a quilt and I can't wait to get stuck in.