I recently purchased a catflap from Catflaps UK as a direct replacement for one which had been fitted many years ago and now had a broken locking mechanism. I was able to remove the broken catflap and fit the new one by copying the way the old one had been fitted. The thickness of the upvc door was significantly large and I fitted the tunnel from the old removed catflap between the inner and outer doors of the new one as none was provide with the new one. No mention was made in the instructions that this thickness of tunnel was available to purchase or even existed. A further quibble was that the plastic push in caps provided to cover the heads of the fitting screws (again salvaged from the previous catflap as they were not provided with the new one), only fitted very loosely in the external screw holes. I tried to remedy the looseness by using 'Loctite' but I do not know if this will be successful at holding these caps in position. Apart from these points to do with 'fitting' the catflap is fine and works well.