HI, my company is bringing to market a bioreactor  which is a machine that consists of several pressure vessels and interconnecting pipework, control valves and a control box. There are external inputs of steam and electricity. The pressure vessels are manufactured by another company and incorporated into our machine. My question is regarding  the presence of the registered number of the notified body. Should the CE labels of the pressure vessels from the third party have the registration number of the notifieed body on them?  Do we also need to get the system as a whole certified by a notified body or can we self certify.

Can I suggest your company invests in and understands the applicable standards - the answers to your question are contained therein tupically in the "marking" section of the appendices.

https://www.gov.uk/government/publications/designated-standards-pressure-equipment

https://www.gov.uk/government/publications/pressure-equipment-safety-regulations-2016/pressure-equipment-safety-regulations-2016-great-britain

Bit more guidance although this is, like most HM gov Brexit guidance, like wading through treacle