Likewise, this heat exchanger thing took hours to remove, clean and replace and it wasn't even the problem so the engineer didn't charge for the time, which I really appreciated.
I just wanted to leave a quick note in case anyone is googling for this same problem. I also have a sabre he25 which packed up recently. Engineer suspected the problem might be a blocked secondary (or dhw) heat exchanger. BE WARNED. The dhw heat exchanger on this boiler is difficult to access...
Yesterday I had a carpet put into my bedroom, and asked for the floor to be covered with hardboard first to prevent 'filtration marks' on the carpet. I would have done the hardboarding myself but I knew the 'pros' would be quicker and having just moved into my flat I wanted to get the job done...