My friend recently had a very similar problem with old 28i junior combi. His red light flashed about once per second. Looking in the manual for that boiler it states several problems, but when he called out the Worcester Bosch engineer he said " it was the water control switch that had gone so he had to fit a new one", this was after testing a few other components.
Unless it's a low water pressure problem in the sealed system (that can be alleviate by opening the filling loop) you will find that you will need the help of a CORGI engineer who is able to test the parts and locate the exact problem.
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