I want to upgrade the 20+ year old shock on my NC35. I've got a rebuilt, late model NC30 shock but also the opportunity to go for a Nitron. Is it worth spending £400 for the Nitron or am I not likely to notice much difference, bearing in mind this is for road only, not track?

If your current shock is shagged then you'll notice a difference on the road, but make sure that your forks are also in good working order and set up correctly

Forks were professionally overhauled a few months ago. So would you go for the Nitron or settle for the rebuilt NC30 shock?

I was faced with the same decision a year ago and I opted for the Nitron R3 for a few reasons.

It was recommended by my suspension guy that I replace the reservoir with another Honda item from another bike that included compression adjustment, which required me to purchase a second hand shock off a wrecked bike for parts. On top of that he recommended that I replace the line to the reservoir with a braided one. I would have also needed a new spring.
All up the cost of rebuilding the NC30 shock that I was running on my NC35 was going to be between $800-$900aud, while the Nitron that I chose came to $1200 and was fully set up for me and is a significantly better shock. I then sold my NC30 shock for $150, which brought the total cost down.

Having said all that, my bike is a track bike - so ultimately spending more money for a high-performance shock was easier to stomach. I suggest talking to a local suspension shop about your options and getting a quote for both.

If you're just road riding, you're very unlikely to notice a difference between the two, so cost could be the biggest difference. I've always found that the front end being set up correctly makes the biggest difference, so given that your front end is set up properly I wouldn't worry about spending too much on the rear end unless the stock setup will hold you back.

Spoondiddly is in this postition just now for restoring my old SP400 race bike I imported from Japan. I want to keep it original so keep original shock but out of interest do you know a suitable bike shock to get a remote res from that includes compression adjustment?

If I recall correctly, a late 90's cbr600rr reservoir was suitable. My 1994 cbr900rr has a shock with a similar reservoir that would likely work also.
Apparently they all have the same thread for the reservoir, however they are made of thicker material which makes it easier to fit a schraeder valve for re-gassing.

When I was at the shop he also brought up a website that included excruciating levels of detail about every oem and aftermarket shock, which included a lot of the information that he used to confirm that a cbr600rr shock worked.

Thanks, wonder what the website was? Would be useful

Looking at the HRC nc23 shock I have the res is very similar to those used on the RC30 and CR range. Not likely to find an RC one cheap but wonder if the CR would work, would keep the look I want too

Faced with a similar issue in 2015 when refreshing my 92 NC30, OEM shock just too soft and old.
After a lot of research, ended up with a shock from Mike Capons, Shock Factory, great advice and built it so it can accomdate the oem 18” wheel or a conversion to 17”.
Quality is top notch, performance fantastic all for a very reasonable 350 delivered.