We bought a new Heatpump for my daughters room. We negotiated the other 3 heatpumps in our home to be cleaned by the same reputable crowd at a discounted rate. 

Bad weather prevented the installation of the heatpump but they did clean all 3 heatpumps on the Friday. They said they would install on the Saturday. Bad weather prevented that as well.

When I got home, my home office, had my Panasonic set to it's lowest temp running at max speed which I turned off when I got home. The entire room was exceptionally cold. Everything was ok, but halfway through watching the rugby (about 3 hours from turning off the heatpump), the RCD tripped, basically killing most of the downstairs area. After some investigations and isolating tests which took around 30 minutes, I found switching off the Panasonic Heatpump at the kill switch in the garage allowed everything to operate correctly. I contacted the after hours number for the heatpump people and got a text saying they didn't think anything they did would cause it but said they would send someone in the morning to have a look. They said to leave the unit off which I did. 

The following morning a guy turned up and had a look, turned on the master heatpump switch and no issues. He then proceeded to pull the heatpump apart looking for potential causes. I left him to it.  He then tells me that a bug was the cause of the issue because it was "probably" crossing the terminals inside the heatpump causing a short. He didn't show me any bug, but upon inspection you would need one hell of a bug to cross these terminals which aren't massively far apart but are recessed. 

I also mention that the heatpump in question is noisy now, like a rattling noise, which the guy takes a look into and ensures everything is secure. Both this and the upstairs heatpump they also cleaned have similar noises since the clean.

He left and I think nothing of it, then I get an invoice for $185 for an after hours callout to resolve a bug issue. 

I dispute the claim saying I don't believe a bug which I was never shown, could have caused this issue, esp since surely the bug would have gotten electrocuted and fallen away meaning it might have tripped once but not subsequently, and that as soon as the guy arrived and tested, it was working fine before even opening the heatpump. 

The company insists we pay and that it was a bug and nothing to do with them. The Heatpump is 5 years old and I've never had a bug inside it before let alone one that would trip the RCD. 

I was never told that a callout fee would be charged. It seemed likely that something done during the service was responsible given we had not a single issue with any of our heatpumps since they were installed. 

The techs notes indicate he showed me a bug which I categorically never saw and wasn't shown.

Not sure where to go from here.