I read some of the previous posts on this problem but I wanted some opinions before I delve into the bike's electrics (of which I am pretty ignorant).
Took the NC35 out exactly a week ago. 50 miles without problem. This morning switched the ignition on and no sign of life, no lights, nothing. To me that means a dead battery. But up until I put the bike away last week everything seemed fine.
I usually plug in the Optimate but this time didn't (it's on the Optimate now just in case it's recoverable). The bike had a recent overhaul by Graeme France and was running perfectly, with new plugs and everything checked (he's a very thorough chap but obviously couldn't have foreseen a rec/reg failure).
The rec/reg is probably the original and the battery is at least 4 years old.
The only unusual thing I noticed was a strange smell, which I was pretty sure was coming from the bike, a couple of miles from home. Could possibly have been a 'burning' smell but maybe I'm mis-remembering that.

If I stick a new battery in it will a damaged rec/reg wreck it? I don't own a multimeter. What d'you reckon?

Without a multimeter you can only start looking for damaged connectors, components etc and the source of the "burning" smell. The rec/reg connectors and the main fuse, located in the starter solenoid, would be good places to start.

or buy a gasket for the stator & pull the cover - any damage to the stator will be fairly obvious..

If the bike had a recent overhaul by Graeme France, I doubt very much he would have left it fitted with an original (prone to failure) Reg / Rec or would not have given the charging system a very good check.

Worth giving everything a good visual check but as the guys say, you can't really check things out without a multimeter.

For my 2p worth I would think that at "at least" 4 years old your battery has just given up the ghost and that only by living on an optimal it has kept it going this long. Fingers crossed it is something simple.

Some helpful suggestions, thanks. I'll whip off the tail section and see if there's any obvious damage to rec/reg connectors and main fuse. I've got a good battery in the NC29, so can switch them and see what happens. I'll also be giving Graeme a call to see what he reckons. Yup, fingers crossed. I'll also be buying a multimeter!

Just had a quick look at multimeters on the Net. Blimey, quite a range, with prices from under a tenner to well over £100. Presumably I'm looking for something at the lower end of the price range, but not some Mickey Mouse gadget that isn't accurate. Any suggestions from those in the know?

I've visually checked over everything I can see under the tail section. Can't switch batteries because the +ve and -ve terminals are the opposite way round (thank you Honda) and the battery cables are too short to reach. No burn marks, melted wires or blackened connectors and the rec/reg looks ok. However the main fuse has blown. I replaced it with the spare which immediately blew the moment I tried to reconnect the 'lid'.
So I'm guessing the battery isn't the problem.
Would a knackered rec/reg cause a new fuse to blow the moment it's connected into the system (with the ignition off)?
Strange how the bike did the 50 mile trip and was running fine up until I switched it off and is now dead. The fuse must have been ok until that moment. What might have caused the fuse to blow as I switched off the ignition?

Disconnect the rec/reg and replace the fuse, if bike starts and runs ok then suspect the rec/reg. The rec/reg is an unswitched connection to the battery, the main fuse will blow if the re/reg develops an short to chassis.

any multimeter about £5/10 will do, just need to read voltage up to 20V, should have 12.3V to 13V ignition off, 14.5V approx when bike running. If going above 14.5V (like up to 17/18/19V then the regulator part has gone). If less than 12V then suggests the stator has gone or the battery has died

Progress! Followed your suggestions..disconnected the rec/reg and replaced the fuse. Didn't blow. Bike started first time! Had a word with Graeme France (and exchanged emails with Rick O). Heavy duty rec/reg is in the post. A new battery was recommended, so that will be my next purchase. A multimeter is also on the way so I can double check. Happy days! Thanks for your help guys.