Thermostatic Cartridge Assembly

The shower started to drip, the plumb shop staff reckoned it was the cartridge.  Then I started looking for a replacement, everywhere I looked, "How much?!" can be heard.

I forked out over a hundred quid for the cartridge and an hour of fiddling, it is fixed. 

The way it was installed, I could not change the filters, oh well.  I still wonder what the glue was for, supposed for seals replacement, but it was not mentioned anywhere in the O&M and the video.