They're a use-once clip, designed for quick assembly at the factory using some kind of high-powered press, to avoid paying someone to stand there for 5 minutes with a screwdriver.
Just lever/bust them apart with a screwdriver (lever against the clip, not the hose), then replace with a standard jubilee clip from any plumbers' place, e.g. screwfix, toolstation etc. Just pick your size:
Free next day delivery for any order over £10 - they're a brilliant company for all sorts of bits. Run by the former founders of Screwfix, who sold out to B&Q then started again.
I had to use loads of them when I changed the thermostat. A pig of a job, by the way.
When hoses get older then you are better off using the pliers to remove the hose clips and then replace them.
If you don't have hose pliers then be careful when you remove the clips. You can twist them off but you have to be careful that the hose it no damaged in the process.