how can i get hot water?

I have an ravenheat rsf820/20, it appears to be about 7 years old not sure.
I only get hot water if i have had the heating on for at leat half an hour!!!
I do not get any hot water if the heating is switched off which is not ideal in the summer. I am lucky if i get luke warm water when i turn my hot tap on. I have to blast my tap on full before the boiler pulls in! the water is still then only warm and usually dies off in a matter of seconds.
This seems to be common problem with this particular boiler....does anyone have any solutions? other than having it replaced and having engineers to come out to each tell me that it is a different problem and charge me £50 each time!!