There have been two instances recently which should awaken you to work done by companies supplying specialist services or equipment. One could assume that, because you have engaged companies who claim competence is certain areas, the you are absolved of the risk. Sorry, this isn't so and the employer is the one who is still held accountable.
In the case of Brinton Carpets (see Carpet Company fined £11,000), one would have thought that engaging Allianz, not exactly a back-street company, to carry out pressure tests, then you would be in the clear. But Brinton were fined over £11,000, alsmost as much as Allianz's fine.
In another recent situation, Company A bought a CE-marked machine from a reputable Italian manufacturer. An identical machine had also been purchased by Company B. A minor accident occurred on the Company A's machine which triggered them to look more deeply into the risks, during which they identified another area. The machine at Company B had already had guards fitted by the manufacturer to their machine in both these areas, but the manufacturer had not seen fit to upgrade the design to include these or inform Company A. Were there to be legal case over this, in my experience, action would be taken against Company A, rather than the manufacturer.
So, you must ALWAYS thoroughly risk assess new machinery and never rely on the CE mark. And you must ALWAYS check that your outside contractors of services for pressure equipment, lifting equipment, fire equipment, etc., checking is actually picking up all the applicable equipment.