I;m about to replace my existing metal cased wylex consumer unit,the
one with the big grey RCD on/off switch which has a penchant for
nuisance trippage,with a new unit which will have an isolator and
mcb,s. Do i really need an rcd though ?

The problem is that whole house RCD units are both prone to trips, and their
consequences, as you've found out.

The best, but most expensive solution, is to use RCBOs instead of MCBs for
socket and exterior electrical circuits. This way, any trip only affects
that circuit, and trips are less likely anyway, because they only measure
leakage on that individual circuit.

The next best solution is to use a split load circuit. This has only a
single RCD, but this only covers high risk circuits, like sockets and
exterior circuits as above. Some circuits (such as lights) are not covered
by the protection.

In either case, a fridge freezer should actually be on a dedicated circuit
and not be RCD protected.

Note that if you have a TT type supply, the circuits MUST be RCD protected.
This is normally achieved using a time delayed 100mA RCD, with 30mA RCD
protection provided as above. Alternatively, RCBOs could be used for every
single circuit without exception.

Christian.