Respect, don't fear, aluminum
First and foremost, don't be afraid of the aluminum wires. North America had a bad time with small aluminum wires (but not large ones for some reason), but did the hard research to find out why they failed. Two rules rose to the top.
- The aluminum wire must land on terminals actually designed and rated for aluminum wire. The most reliable are what I call a "lug terminal" which has an aluminum body, a hole to put the wire in, and a setscrew to clamp the wire.
- The screw torque must be set correctly with a torque wrench to the manufacturer specification - yes, even on the small stuff! The eye-opener here was extensive testing of master electricians that showed they were extremely bad at setting torques by feel.
Europeans and British positively love lug terminals, but the key is making sure your lug terminals have an aluminum body. Those are "universal donors" which cheerfully accept both aluminum and copper, due to thermal differences (i.e. aluminum's larger expansion rate works favorably when it's the lug and the wire is copper. The reverse is a disaster, and that's what you must avoid.)
Of course most people simply grab any socket or switch they can get, and don't use a torque screwdriver, which creates the risk with aluminum.
So become a master of identifying either receptacles and switches rated for aluminum wire, or finding a Europe-legal aluminum-body lug splice connector such as North America's Alumiconn, which accepts both aluminum and copper and allows you to "pigtail" to copper-only devices.
Failing to torque to spec, and improperly rated terminals, both have the same potential bad effect: a "series arc fault" meaning normal usage current is jumping across a small gap, making a very hot arc. Your overcurrent device won't detect this, and neither will the RCD. However the North American experience is that properly rated terminals and torquing to spec takes care of this.
It's not the wiring in the walls
Often when you can't find the problem in areas you can see, you start thinking the problem is in areas you can't see. Try your best to put that out of mind. Damage inside walls is unlikely unless there has been recent construction or nailing into walls. The problem is overwhelmingly likely to be inside accessible points such as junction boxes - this is largely the purpose of junction boxes.
The thing about UK ring mains is they are typically 32A or 7680 watts (4 times the capacity of American 120V/15A circuits)... which means they can power many rooms - a large part of the house. So that means quite a lot of hunting.
To identify which things are on that circuit, turn it off.
RCD trips have totally different reasons than overcurrent
You actually are displaying two very different symptoms here.
An overcurrent or short circuit trip is indicated when a plain breaker trips. Breakers don't have a way to tell you which one occurred.
An overcurrent trip is simply due to over-use of the circuit with too many appliances running. This can be determined by looking at the nameplate data of every appliance which makes an appreciable amount of heat, and totaling up the amps of the various loads.
A short-circuit trip is caused by a hard dead short between a live wire and a neutral or earth wire. This could be happening inside an appliance, but it will generally blow the fuse in the appliance plug before blowing a 32A breaker. So more likely it is in the house wiring somewhere (again almost certainly in a junction box). Typically this may be a frayed live wire contacting a frayed neutral wire, or a bare earth wire contacting a live wire or live terminal.
I understand you have an RCD module which is an RCD only, and not also a circuit breaker monitoring overcurrent/short circuit. Having that functionality separate is very valuable, or at least having indicators on the device to show the reason for the previous trip.
The RCD has a completely different operating principle. As you know, electric current flows in loops. The "outbound" current should always be exactly the same as the "inbound" current. The RCD compares those two currents and assures they are the same within 30mA. If it's not, some current is going AWOL and may be shocking someone or starting a fire.
This can be a tiny amount of leakage from live to earth or other earthed point like a water pipe. Or it can be leakage from neutral to earth, and that's something most people don't think about! However, neutral-earth leakage only arises when loads are active on the circuit. And depending on the nature of a neutral-earth fault, it may only arise when significant loads are active on the circuit.
Now the nice thing about UK installations is that for the overcurrent or short circuit trip, we can cross off plug-in appliances. They all have fuses in the plug which will blow first. So you can focus on each switch and receptacle in the ring main. And simply work your way through them, one at a time, pulling them out, inspecting them for deterioration and to assure the terminals are aluminum rated, and torqued to specification. Or, pigtail them off of an aluminum-rated splice.