Stelrad Ideal NF 70 Hot water pressure ?

Hi all,

I'm trying to help out an elderly neighbour, they have the above boiler fitted in their bungalow. The problem is Hot water pressure (lack of it) if you turn the hot tap on in the bathroom sink you get a slow flow of water, however, if you turn the bath hot tap on the sink flow stops? the only way i can get hot water to flow from the kitchen tap is to bung both hot taps in the bathroom, then you get a steady flow in the kitchen.
Would i be correct in saying this is a lack of head pressure? if so is there a cure ?

Many thanks.