RCD tripping under high load on 2 circuits

I’m in desperate need of someone with great fault finding knowledge. I have a dual rcd consumer unit as seen in picture. The left hand side rcd trips when anything with a high output like a tv for example is turned on in any sockets protected by the left rcd. The left rcd also trips when trying to switch on any light switches also protected by this rcd. The left rcd also trips if I use an appliance in the kitchen which is protected by the right hand side rcd, the appliances still work in the kitchen but they trip the left rcd and the rcd will not reset until I turn off whatever I’m using in the kitchen. I have had an electrician come round to investigate who left still scratching his head and told me that a full test would need to be carried out on the consumer unit as the faults were not pointing to one specific thing as 2 circuits are tripping the left rcd and not just 1. My house is 4 years old and I have owned it since new, one problem I have had since owning the house is the bath leaks onto the living room ceiling. I’ve had the builders out twice for this but that problem still persists. It has been like it for 4 years so I’m not sure if it has finally caused a problem? To summarise, any high output appliance used in the sockets protected by the left rcd trips the left rcd and also the light switches protected by the left rcd trip the left rcd. Everything in the kitchen works but using them causes the left hand rcd to trip. I have switched the rcd units with each other to eliminate the possibility of an over sensitive rcd but it made no difference so I have now swapped them back. I have checked the wiring on the sockets and light switches protected by the left rcd but couldn’t see any damage/loose connections. Any advice would be greatly appreciated as I’ve been trying to diagnose this problem for a week without any proper testing equipment. Kind regards, Simon