Gas safety inspection

Following years of gas safety inspections and a happy tenant, I asked a firm I had not used before to do the regular gas safety inspection. At the time of booking I asked for the Gas-Safe number and they asked me what I wanted to know that for. They did however eventually tell me the number and I checked it. A few days later, I received a message from an engineer sent by the firm. He had found a fault and isolated the boiler, because he said the flue needed re-sealing. Once a repair was done, he would do a re-inspection. I asked why he did not do the repair or call me while he was at the premises, and he said that even if he did that kind of work, he certainly wouldn't have time - I would have to do it myself or get someone else to do it. Accordingly, I got on with arranging the repair.

The day after the initial visit, the gas engineer sent an angry email protesting that he had not yet been paid, stating that I should watch my step because the fault he had found could get me in serious trouble. This sounded rather like blackmail. I was amazed by this attitude and simply paid the bill as normal.

After paying the bill, I was expecting a re-inspection. Instead, I received a further message from the gas engineer stating that actually I should just take a photo of the flu and send it to him and he would issue a new gas safety certificate showing a pass.

By this point I lost confidence in the firm so I paid another registered gas engineer to inspect and issue a fresh gas certificate.

Does anyone have any advice regarding the first gas engineer?

Thanks in advance.