Hard to say without seeing it, as it depends on the state of the rest of it.
Certainly there is no doubt that new sockets in the conservatory (well, anywhere actually), and any new lighting wiring , will need RCD protection somewhere to be regs compliant.

Existing work has a much lower bar to pass, generally only needed to  meet the regs at the time it was installed, providing the changes since then do not mean it would nowadays be considered an immediate danger. 

(If you would like a more homely example, driving a  1970s car is not illegal if it does not have seat-belts, of course it is not as safe, but not considered such a great  danger it must be banned, but you cannot market a new one like that ..)

The current advice is that wiring  pre-dating RCDs is not an 'immediate danger', just firmly an 'improvement is recommended' Do be aware though that over time the advice about this sort of thing moves one way only... stricter ;-) One day it will be disconnected....

The fastest way to do this may well be a new consumer unit, but a mini CU with modern RCDs in it just to cover the new stuff supplying the conservatory might be an option, if you have somewhere sensible to put it.
But that may not work out any easier, and if there are other aspects of the extant building wiring that need attention, such as rubber covered cables, or lighting with inadequate earthing, then this will all need correcting too.
By all means post a bit more description and we can advise better.

Mike

No, I do not think that your sparks is being over-cautious. Updating the CU to a safer option is sensible just as, to use Mike's analogy, a modern car would be safer than a 70s (or in fact 60s) classic. Both cars may get you there in the same time and in equal comfort, but it is when things go wrong that the modern one is beneficial.

Doubtless you bear in mind that the sparks will make a profit from the installation of a new CU.

Extending a ring is not notifiable and there is no legal requirement to issue any form of certificate. However, a minor electrical installation works certificate is appropriate. This would mention the absence of RCD protection.

If your "garden room" will have sockets which could be used for equipment to be used outdoors, having RCD-protected sockets would be wise and if the do not have it in the CU, they can have it built in. (The latter would have satisfied the 15th Edition of BS 7671, which was current in the 1980s.)

errata

the seat belt law in the UK came in 1966 that cars had to have provision for them in the front.
Not using them in cars so equipped became an offence in 1983 - about the time I was learning to drive. It now feels quite odd now to visit a country where they are not required.

RCDs for new sockets supplying equipment for use outdoors came with a regs update  in the mid 1980s, and another in July 2008 extended this to pretty much  all new general use domestic sockets.

There are quite a few non RCD installations around still but they are certainly getting older.

Mike.

Any new CU probably want to contain SPDs as well as RCD additional protection.

I disagree that the existing CU must be replaced - only the new work will be certified - so adding RCD protection at the point where the new work starts would be an option (or indeed anywhere upstream of that point). Likewise SPDs could be added in (perhaps in an extra enclosure, probably near the origin would be best). More than likely replacing the existing CU would be the neatest and safest solution, but not the only one.

   - Andy.

It is staggering the number of places i go to where people spend thousands of pounds building extensions or getting kitchens replaced and want to bodge up the existing electrical system instead of doing it properly. 

Well, you know what they say...... as long as the lights turn on and the kettle boils.... what can be wrong?...

As long as the existing circuits continuity, insulation and earth fault loop test results are okay, the new additional sub circuits comply with BS7671 2018:A2 standard (a type A RCD fused spur supplying radial sockets and another for the lighting), all should be good.

It would be advisable to have a new consumer unit though.

When was the last periodic inspection and test (EICR) carried out?

Why can't you just extend the existing ring?  Extent the loop ring from where the fused spur is currently. this assumes the fused spur in on the ring. Peter

It was my assumption that "just" extending the current ring as it already was extended would be okay but the electrician is adamant that this could not be certified. Hence the questions. I am awaiting his quote to replace the box.

Why can't you just extend the existing ring?

That's certainly an option - although it would preclude adding RCD/SPD protection at the start of the new work - and you'd have to check it didn't throw the ring too much out of balance, or increase Zs elsewhere too much.

   - Andy.