OK, so we've been having these trips for 10 years now and you say I simply need to change the main trip RCD to 32 amp ?
The smart meter was installed only 2/3 years ago.

That is not enough for 5.75kW potencia and you should change it to a higher rating. Very easy and cheap to fix. I hope this will cure your problem.

I am very surprised your electricians did not ask about this?

Hola

Over the years, I have had the electric trip out for no reason. However earlier this year it became weekly or more frequently so I had the main trip switch itself changed and had no further problems.

Davexf

I am informed that 25 amp should be sufficient for 5.75 kW potencia (= 23 amp).
25 amp is on the limit for this supply, but it isn't our consumption which causes the trips (so I am informed by an electrical engineer).
However, I'm interested to know if you uprated your fuse or merely changed it for a new one.
I guess they can get old and tired, but our trips are no more frequent now than they have been for 10 years.

Usually if the RCD trips without any of the other circiuts tripping its an earth leakage/insulation breakdown problem. We had spate of these when one of the elements in the cooker was on its way out. As it's so intermittant its probably not a practical proposition to isolate each circuit in turn to identify the culprit. Most common device after the cooker is an immersion heater. A good tip was the faulty RCD itself. If you can't work it out then you will need a qualified electrician with an earth leak detector (bridge megga). Don't hire the one who says it could be anywhere in the neighbourhood as the RCD works on the downstream side.

I can't see how a sparks would tell you the problem could be somewhere in you neighbourhood , your RCD protects your circuits and no one else's.

have you tried isolating a circuit at a time when it's being trip happy by flipping the MCB, do you have many outside electrics , lights etc as water from rain or condensation will trip a RCD,

But 5.75kW isn't 23A @ 230v (which is the mains voltage in Spain), it is 25A. Divide the watts by the voltage to get the amps.

So your 25A breaker is right on the edge, and the advice you were given earlier, to uprate it, is the correct advice.
Well if something causes a voltage spike then the current would also increase and could trip the breaker, especially as it is literally on the edge of the required value.

The RCD is not a MCB , the amps are a rating for the maximum potential to run through the RCD it's not as with the MCB a current rating for it to blow, presuming the RCD is tripping when they are away as has been said they are not drawing huge current anyway, the RCD simply measures leakage to earth

Ooops, yes.

I have that issue with one circuit. It only feeds two pairs of sockets, and I have replaced both sockets and the RCD breaker, and the problem still persists. My electrician says it is likely a manufscturing flaw in the cable or a bad instalation - a nail or screw through the cable somewhere. But I am not interested in having the house torn apart to have a new cable installed.

Oh - if only it were "trip-happy". We could have solved it years ago.
Trips occur day or night, winter or summer, rain or shine, everything connected, nothing running - and can be 2 or 3 times in 1 day or not for 2 or 3 months.
When we go away, we empty the freezer, unplug it and the immersion heater. But we almost always return to a "tripped" supply after a few days or week - but sometimes not.
Its that erratic.

This seems like a very logical explanation and tends to support the theory that the fault is not necessarily "downstream" and could be the result of external influence - i.e. voltage spikes, which I believe are common here in the campo especially.

If you can't find a fault then maybe it's a faulty RCD ? , also on your consumer unit do you have 3 phases coming in?

We sometimes get the RCD trip, in fact it went yesterday but we had a few hours of very heavy rain so could have been a mains spike but that's the first time in around 9 months , any priority services ie, alarm I've put on a none RCD protected circuit, still protected by MCB but not a problem if we are away, we live in the UK so this is often the case

Apart from the RCD you will most likely have a number of fuses, mcb's protecting various curcuits.

Try turning off each circuit in turn for as long as possible ( days ) and see if the RCD still trips
If it does then the turned off circuit is unlikely to be the suspect, if not you have narrowed the problem down.

Hopefully there is only one suspect circuit.

This is not a practical option as we can go for weeks/months without a trip.
As for incoming phases, I have no idea.
Its a regular 5.75kW supply to a house in the campo - as far as I know.

5.75 is defo single phase.

ISTR 5.2 is 3-phase