Plumbing?

I have a Potterton Flowsure oil fired combination boiler, I was supposed to have the Flowsure Plus with storage but the plumber said the Flowsure would be ok. Well the heating side is no problem works well.
The hot waterside how ever is very poor, if we need to run the bath, we turn the tap on turn it down to a dribble put the kettle on, read the paper and eventualy we have a bath nearly full of hot water. Any ideas? can we fit storage? The boilers only 5 years old
so don't really want to srap it. Its been a frustrating 5 years :-(