I mainly make bespoke design for clients. I understand the copyright rests with me? What should I be particularly aware of with one-off-designs?

When you create a one off design for someone they will have the right to use it but they won’t own the copyright unless you assign it them. It really depends what bargain you have struck with the client and also what you want to do going forward. If you assign the copyright then you can’t base future designs on that work. If you aren’t bothered then it may well be a way of creating an extra income line; selling exclusivity if you like. If you do want to base future designs on the work as it is, for instance, a signature piece, then you give your client a licence. The thing you should have is clear terms of business so that what clients can do with the work is agreed beforehand.