Too much pressure on Worcester 28i Junior?

My decorator took off a radiator and heating would not come back on. I repressurised, bled all rads and eventually red light came back on, but pressure had dropped a little. I turned boiler off and added a little more water. As soon as I'd turned the tap off the pressure rose quickly to 3.5 bar. And of course I daren't use it - do I open a valve on a rad and drain some water out until pressure drops?
And considering I've now added a lot of new water to the system do I need to add something to stop the rads rusting? And how do I do that, or do I ask a plumber to do it at annual service?
Hope someone can help, we're going to be shivering here until I get an answer, I thought the boiler was going to blow up when the pressure shot up!