Thanks for your comments. The original plumbing company sent someone back today, but they sent a different plumber. I'm told the part changed back in December was called a 'Pressure Reducing Valve' (see image attached). I explained to today's plumber that I was charged £500, and he pulled a face ie) wide eyes, look of shock! I laughed, and told him I also pulled the same face when I was charged £500. He then looked a bit awkward, and tried to suggest the first plumber changed another part as well (although he couldn't tell me what), but that's not what I remember being told back in December. In December, surprised at the cost, I requested a breakdown of costs, and a receipt, but nothing was emailed to me. Today I again asked for the breakdown, but I'm now being told by the office manager that the company has just 'changed owners' and they have no invoices.