Different point of view:
What's the efficiency on the boiler? 50%
I guess your gas bill is about £800/year? That's £400 up the chimney a year.
A new system boiler in the same spot with a powerflush, controls etc and a 12 year guarantee would cost ???
Do the math. How long until it pays for itself?
It is not that different. My first post made a similar comment.