Things are edging along on the CVC. I am starting to put back the gearbox cover and dash trim and I can see the O ring location fairly easily not necessarily reachable easily but there is no sign of leaks. So should I leave well alone or do them just in case? The car has been sitting for about three years unused.

If you have easy access because of the no dash it would be a pity not to do it ... at least that was my thought when I made mine - no leaking, but ....
I also silicone the way around the rings - you never know ...

The original heater o-ring is natural rubber and would have been already replaced by this time, so I would guess the one in is probably nitrile rubber by now which last longer.
If you don't know the history, if it were me I would put a new one in. Also change the retaining screw for a stainless allen bolt.

It isn't all that bad - once the dash is out and it's all apart, it makes sense to do it. I replaced the core on mine and put it back together having not tested my 'repair' on the passenger heat motor. Then spent the summer wondering why I didn't do it when it was in bits. Now I need to take the dash back out before next summer (or see if it's workable through the glovebox hole but probably, it isn't). The only part of the heater job that was looking to be problematic was getting it out from the metal frame in the centre above the transmission tunnel, turns out the large bolts (4x) that go into the floor will allow enough wiggle room when loosened to get the top unbolted and pull the top half of the heater out and off from the a/c bit. Quick edit - looking at my photos the entire silver-grey middle frame came out leaving only the car-coloured bits.

For the sake of a couple of quid, while you've got access, replace them. RAVE says to lubricate with anti freeze rather than try to seal them with silicone. They work by allowing the two pipes to move in relation to the matrix so they need to be soft. Usually when you take them out they have gone square section and brittle. I replaced mine not long after I got the car and it's still fine after 10 years and 200k miles. I've replaced O rings (as a matter of course on any car I've worked on unless I know they've been done recently) and blend motors on various cars at least 10 times but I've never taken a dash out.....

I replaced mine quite easily.

Coolant was pissing out of them mind you..

A long screw driver and some patience.

OK my path is set. Whilst hoping for "nah it'll be fine" I was expecting "get on and do it."

Thanks everyone.

Do change them, I would not use silicone myself.

All done and really not too bad other than a crick in the neck. The old ones looked ok but I changed them anyway and just put a bit of RRG on for luck.

RRG?

Red rubber grease

Ahh, raspberry jam grease, good stuff. My tin must be at least 35 years old and still half full....