How can I fix the hot water flow from my Potterton Puma 80e?

Potterton Puma 80e boiler. The boiler does not produce enough hot water for a bath. It runs a sink-full in the kitchen but cools when a bath amount is demanded; the shower at the bath is barely warm enough. The central heating seems to be fine. The control panel thermostat settings are at max. The boiler is only about 4 years old and so unlikely to be blocked by scale. Is there an internal thermostat that needs to be adjusted or replaced? Can anyone offer advice please?