How can I fix my Vaillant Combi boiler VCW 18 T3W FH?

Whilst using the shower, the hot water cut out. The boiler failed to fire up for water until I'd left it for about 10 minutes.
A service engineer has been round and 'diagnosed' that all it needs is a service and a chemical cleaner to be put through the system. Central heating is working fine. Any suggestions? Do you think the service engineers fix will work?! I'm now having to have the world's quickest showers to avoid being frozen when the hot water cuts out!