Things estate agents do - not always honest

Letting Agents

I recently had a gas safety certificate done, arranged by estate agents, only because tenants were due to move in the next day. The one I had done, only included boiler, not gas hob. The estate agent charged me £90 for it, however they failed to send me the certificate and only did so when I kicked up a fuss. I got an invoice with name of estate agents on it, listing their fees, ie find tenant service, gas certificate, however, there was no invoice from tradesman, which I wanted. This did not seem very honest to me and I had bad vibes, that there was something dodgy going on. I then phoned the gas engineer, I got his contact details from gas safety certificate which estate agent eventually emailed me. I pretended to be a potential customer, not letting on that he had already been to my property. I asked him how much he charges and was told £55. The estate agent was charging me £90, in fact ripping me off by pocketing £35. Surely this cannot be legal and allowed?