Unvented hot water system losing pressure

Hi, I need a heating engineer to come and investigate why our unvented hot water system keeps losing pressure and fix it. I have had a recommendation for someone local and they have excellent reviews, but when I asked them to give me a rough idea how much this will cost (excluding any parts), they advised it would be "just a call out charge of £84 then go from there and keep you updated to what we find as we go". This is not London, I'm in Worcester area, so was very surprised by how expensive the call out fee is, plus it all sounds very vague - like I would be signing a blank cheque! Please could you advise what I should expect a gas engineer to charge etc. for something of this nature and whether this guy's response sounds reasonable? Many thanks!