GDPR, Any experts in the house?

I am intrigued that you are willing to attend to your customer’s other needs whilst being employed only to service/repair those sold by your employer. That would be grounds for breach of your terms.
For GDPR, important to track that with BREXIT, new UK Data Protection Act came into force in 2021 to amend the 2018 change. More details at:
Chapter 3 | UK GDPR

My employers don’t mind. We are the only UK manufacturer for the type of machines we do and we probably have 95% of the market. I don’t work on our European competitors machine. I will however work on other machines that do a different job to ours. It helps keep us sweet with our customers. All of our customers have paid service contracts, when I’m on site I am there for the day. My time is included in that contract so neither the customer or my employer lose anything.

Probably already answered. Anyone collecting information needs a privacy and or GDPR policy of some sort in regards to how they save and store the information they collect and who they are sharing it with.

You could also submit a FOI request for shits and giggles.