I worked it out that it wasn't merging when I used merge by distance. Not sure why tbh. I resolved this by going into vertices mode, selecting two vertices at a time and merging at centre instead. Must be the way I've done my model somehow but it's all good now :)
Hi Paul Umbrinox ,
If the Edges aren't Merging by Distance, it could be that they are too far apart; you can then increase the treshold (Merge Distance) a bit.