Worcester 240 Boiler. Water is hot with tap half open, cold when fully open?

I have a Worcester 240 gas boiler. Recently, I've noticed that if a hot water tap is partially opened there is plenty of hot water. However, when the tap is opened fully the temperature gradually reduces to not much more than stone cold. This has become a problem with the shower (Aqualisa Axis mixer type) as it now doesn't reach a temeperature where it can be used without grimacing!! I've watched the flame inside the boiler and it doesn't seem to take account of how much water is flowing through it. As soon as there is demand, the main burner kicks in to a level. The flame remains at that initial level regardless of changes to flow. Should the flame adjust to match the demand? If so what might be causing it to fail?