When i turn the tap on for a bath the water is hot for a while then goes cold?

When i run a bath the hot water keeps running cold then back to hot again, its really annoying me, same things happen to the sink in the bathroom but everything ok downstairs. I have a worscester boiler installed and have checked the pressure gauge which reads 1, also it sometimes makes a funny gurgling sound. Boiler fairly new- is there something that i could do to fix it or am i best calling out an engineer. Central heating ok and ive turned up the thermostat to 22 degrees. Hope it's not going to be to costlly as the warranty has just run out.