Vaillant Turbomax 824 Problem?

Hello,
I have a Vaillant Turbomax 824 E and have managed to sort out problems with it myself in the past by looking at forums.
Slowly but surely a new issue has developed, which I've worked out is a matter of needing the heat exchanger descaled. I've read this is one of the easiest jobs in the world, but for the life of me I can't see how to remove the thing.
I'm hoping someone can let me know what cost I can expect to pay a plumber/boiler technician to come descale it for me in East London (Shoreditch/Hoxton area). And I'm also hoping someone can let me know how to go about finding someone who's really honest and straight up. My only plumber friend doesn't live in London - or work with boilers.
Thanks!
Emily