Glow-worm second annual service

My relatively new Glow-worm boiler (only 2 years old) has already developed the defect of producing a sudden stream of cold water during showering and, less uncomfortably, during washing up. It can last for what seems as long as 2 minutes during both uses.

During the recent annual maintenance I mentioned this defect to the serviceman who said it could be scale in the shower unit (on the wall), which he said would require inspection by a plumber. The other location - when washing up - is the kitchen where the boiler is situated. I told him this was a fault which generally occurs in older machines as I remember it occurring with my previous boiler which was then 10 years plus.

Today I looked at my previous posts in this forum when in my lengthy writeup I included 2 items: Descaler and Mains Chemical flushing. I don't think the serviceman used either at this time. I hadn't remembered this previous post or I might have asked him to do this.

I would appreciate your advice whether these additives should have been used for this second service and possibly every further service and whether they are useful to prevent the cold water problem.