Hi Graham, I think you nailed it when you said the smaller 4s and 5s are useful in the faithfull range, I used my record no.4 and the axminster no.5 on some 3/4" pine endgrain the other evening, it was an absolute pleasure to use both planes, there was no discernible difference between the 2 with regard to finish/ease of use. the axi made short work of removing the cupping on a pine board too, with or across the grain was no issue at all, all in all I'm very pleased with my cheap no.5.
I'm going to look at the axi/faithfull block planes, I get the feeling you can go particularly wrong with something that size, again, with your comments, I wonder if the length/weight of the plane has more to do with the errors you saw than the actual castings? 1.5mm bow is a lot, from what I've seen of the record no.4 and the axminster no.5 there's no discernible error in the sole, if there is it's negligible, either that or I just wouldn't know a decent planed bit of board if it jumped up and smacked me in the face.