How can I fix my Worcester heatslave 12/14?

Our boiler packed up in the night and when we went to check it the temperature gauge was right up.
We topped it up with some water and the temp gauge has gone down and now seems to be working again but the boiler is making a funny whirring noise.
The gauge has started to go up again tonight.
We are now toppping it with more water and have turned the heating down. Do we just need to add more water or is there something wrong with it?
We only had it serviced a few months ago and we had oil delivered last week!