Ideal Icos HE15?

We moved into this property Jan this year and have had no problem with the heating/hot water, however we have found in the past week when the boiler is starting up it gives out a very loud banging noise, once it has fired up ther is no problem and the heating and hot water is working ;fine this only started about a week ago we feel it may be a pressure problem but do not know how to repressurise any ideas