A 400 went for £3500 last year but one just after that was I think about £2700. I would say about £2500-£3000 for a good road ready 400 is about the going rate personally. Rough but needing restoration I would put in the £1200-£1700 area. I paid £700 for my 400 (not running) back in 2014, but prices have gone up as you know the last 5 years. I did get new shocks (splashed out on YSS piggy backs) and did the carb clean and O ring kit myself (use
http://Litetek.co if you do get it)
I just got a 250 and paid £1800 for it from DK - they had 3 and all went for £1800 - supply and demand - they don't come up often so I paid the price. It needs really a full strip down recommission as well, but I could use as is if I wanted.
Shocks probably need replacing really, you can get the cheap Chinese ones if you so wish, but I preferred something better (hence the YSS) but I might try the Gazi ones I have seen on Ebay for the 250 (they are about £300). You can get replacement CDI for about £100 (assuming taxes), but the lock set will be the hardest to get as Fossie says, as it will have to come from Japan more than likely. Exhaust as original could be rusty and looks a little tatty anyway, again you can get new ones from Japan (expensive) but not the UK (unless you get one made).
The 400 is a great bike to ride though, I love mine and ride it more than the 1100. I took it to Denmark and Germany for the EuroKat rallies. I assume you want to buy to restore? The other option is several people have bought the 400 and then installed a larger motor in it (I think one guy did a bandit 1200!) - if you wanted to do that then pipes, ignition, cdi would all change anyway.
Bike is still there but does have a Make Offer button on it, it's got to be worth £1200-£1300.