How to reduce the pressure on boiler?

The top half of the radiator in the attic bedroom wouln't heat, it has been flushed out this summer and still didn't heat up so I increased the pressure on the boiler to see if it made a difference which it did, however the pressure was much higher than it should have been so I've tried reducing it by bleeding the radiator. It reduces but then shoots up high again. The boiler is an Ideal Isar HE30. How do I reduce the pressure if not by bleeding the radiator?