OK
The Memera 21 is quite old, and although a very good brand and good for its time, not up to modern standards.
There is a faint chance the RCD is out of spec, but it would be quite costly to have it tested and replaced.
more likely the problem is that you have the whole house on a single RCD. What happens is that you get small leakages on every circuit, especially from watery appliances or those with heating elements, such as kettles, washing machines, immersion heaters, boilers and their pumps; and also from computers, and from outdoor things like lighting or fountains which might get a bit damp. What happens next is that all these little leakages add up to to (say) 15 mA. You then get a small additional leakage from another appliance, and although it would not alone be enough to cause a trip, it brings the total up to enough to trip the RCD.
For example, in my own house, the ground floor sockets (which includes an old washing machine, garage sockets and outdoor lighting) has a background leakage of 5 to 15mA. Very occasionally it will trip (I have a separate RCD (RCBOs actually) for each floor of my house.)
What will probably happen in your case, if, as is likely, you have a general level of background leakage, is that even if you buy a new fridge-freezer, something else will tip you over the tripping point from time to time. It might be an electric cooker or an immersion heater.
A better solution would be to replace the old Memera 21 with a Memera 2000 consumer unit with RCBOs for each socket circuit, and one RCD for everything else, or, better, an RCBO for each circuit.
You cannot fit RCBOs to your old CU.
Unfortunately the RCBOs are more than £30 each
and fitting a new consumer unit has to be done (in England and Wales, I don't know Scottish laws on this) by a person who is a member of a scheme that can carry out tests and issue certificates. It is likely to cost some hundreds of pounds
Ask around friends, neighbours and colleagues to see if the is a recommended local electrician, but before asking for a quote, find out (someone on here will know) what qualifications and memberships he needs to have. An electrician who is properly qualified will not mind you asking for evidence, he will be proud that he is qualified.
MEM is a very good brand, so I'd stay with that. It is particularly popular in industrial and commercial environments as it is very reliable. Their metal enclosures are stronger but more expensive than plastic ones. Their latest CUs are the Memera 2000AD range, but the squarer Memera 2000 range is IMO stronger and more roomy inside. The RCBOs are taller than MCBs so need more space. The internal fittings are the same in both ranges.
