Thanks for the replies, all really useful!
They were old fashioned Bronze turkeys, not one of the modern heavyweight breeds, and I've always bought that type too, so it wasn't the shape so much as the actual finished weight that was disappointing. Interesting that you've had a similar experience Buffy! And I'm glad it's not just mine that weren't very interested in food of any kind!
I am having some fencing renewed in the spring, so am thinking of using a different shed for my turkeys next time, a bigger one (which requires some work to make it fox and rat proof) where there is space to provide an outdoor enclosure for them. That way, they won't be totally free range but will have access to a safe outdoor space and so will also have their own feed. And I will try and get them earlier too, but it will depend when poults become available locally.
I am also very tempted to have a go at breeding my own, provided they can rear their own babies. Do turkeys make good mothers? Or could I use a broody hen, like with my ducks?