Help with hot water feed on my Potterson Performa 28 boiler?

Hi,

We have a strange issue in our house where the hot water is working, but not for every sink in the house. For example in the bathroom the sink will only run cold, but if you turn on the shower on, the sink will then get hot.

Likewise in the kitchen it will only run cold but when you turn the hot tap on in the utility room, the water gets hot in the kitchen.

Any ideas?

Thanks.